COS Engineering S6/LPS1 on SFP

by Dawid Grzyb / June 28, 2026

Every now and then a review deserves a second chapter. My original encounter with COS Engineering’s S6 lacked the hardware necessary to evaluate its optical SFP interface. This time, that missing piece of the puzzle is finally in place. Enjoy!

No stone left unturned

Optical transmission is nothing new in digital audio. The familiar Toslink interface has been around for decades, yet today it occupies a niche role compared to USB or coaxial S/PDIF. Network audio, however, is a different story. Here optical links have recently gained traction, and for good reason. While conventional Ethernet over RJ45 already ensures bit-perfect data delivery, it does so via a conductive copper cable that inevitably forms an electrical connection between network devices. This is where SFP enters the picture. Unlike RJ45, an SFP cage equipped with optical transceivers converts electrical signals into light, sends them through a fiber-optic cable and converts them back into electrical form at the receiving end. Although the Ethernet packets remain exactly the same, the transmission medium changes fundamentally. A fiber link provides complete galvanic isolation between devices. No current can flow through glass, which means the connection cannot conduct ground noise, leakage currents or high-frequency interference. Unlike copper Ethernet cables, which may also behave as antennas that both receive and radiate RF energy, optical fiber is inherently immune to electromagnetic interference.This distinction is particularly relevant in high-performance audio systems. Modern streamers and network-enabled DACs already employ sophisticated buffering and error correction, so audible differences are not about improving data integrity. The bits arrive intact over either interface. The real objective is to minimize unwanted electrical noise that can accompany those bits and ultimately reach sensitive digital and analog circuitry. By breaking every conductive path between upstream network equipment and the playback device, an optical link removes one of the potential routes through which high-frequency contamination may propagate.

Of course, fiber is no magic bullet. Optical transceivers themselves are active devices with their own clocks, regulators and laser drivers, so implementation quality remains every bit as important as it does with conventional Ethernet hardware. Still, when executed properly, SFP offers something copper simply cannot: complete electrical separation of network components without compromising bandwidth or protocol compatibility. That alone makes it an intriguing proposition for computer audio, where reducing electrical pollution often proves more meaningful than attempting to improve digital data that was already arriving error-free in the first place.The folks at Taiwanese COS Engineering understand this distinction well. Their S6 audiophile switch, which I recently reviewed, has featured an SFP cage alongside its conventional RJ45 ports from day one. My review sample, however, arrived without the required optical transceiver installed, rendering that interface unusable. To add salt to injury, I didn’t have a single SFP-equipped streamer or DAC on hand anyway, so even a populated slot would’ve remained just that. Consequently, one of the S6’s most intriguing features had to be left unexplored, and I made no secret of that in the product’s original review. Fortunately, that wasn’t the end of the story. Shortly after the article went live, COS Engineering’s one and only Stephen Gong got in touch and suggested we revisit the S6 once the missing pieces were in place. Since SFP was still uncharted territory for me, accepting that invitation required very little persuasion. It presented the perfect opportunity to find out whether the theoretical advantages of optical networking translate into meaningful gains in a real-world high-end audio system—or so I thought. As it turned out, the reality proved rather more nuanced. But I’m getting ahead of myself.

Fast forward to this year’s High End show in Vienna, where Stephen Gong exhibited in a cozy room shared with Lu Kang Audio and Kojo Technology. During our conversation he mentioned that, once the event was over, he could send me COS Engineering’s latest D10. I had already reviewed the original D10 back in late 2020, where it impressed as a DAC, preamplifier and headphone amplifier. The newest version retained all those capabilities while adding a fully fledged network streaming platform complete with an SFP input. When the package eventually arrived, it contained not only the D10, but also a pair of SFP optical transceivers and two fiber-optic cables of different lengths to establish a complete optical Ethernet link with the S6. Everything I had been missing during the first review was finally there. The game was afoot.At first I assumed this exercise would boil down to a straightforward comparison between conventional RJ45 Ethernet and an optical SFP link within an otherwise identical setup. Whether I simply wasn’t paying close enough attention in Vienna or jumped to conclusions is anyone’s guess, but that wasn’t the case. The D10 Stephen had sent me featured a streaming module equipped exclusively with an SFP input. Its RJ45 counterpart wasn’t part of the package, which meant there was no opportunity for an apples-to-apples comparison between the two interfaces on the same hardware. No matter. The actual purpose of this exercise revealed itself soon enough once all the pieces fell into place. The two supplied SFP transceivers weren’t included by chance. One occupied the S6’s SFP cage, the other the D10’s streaming module, with a fiber-optic cable bridging the two. That established a fully optical network link from switch to streamer without a single copper conductor between them. Since the D10 now handled streaming and digital-to-analog conversion alike, my trusted Horizon360 DAC was granted a well-deserved break from active duty. Just to state the obvious, everything downstream remained exactly the same.

That setup alone would’ve allowed me to sit back and simply enjoy the music, but it wouldn’t have provided much useful context. To make this exercise genuinely informative, I needed a meaningful point of reference. The solution was straightforward, and realistically the only viable way to carry on. My Innuos Statement Next-Gen remained in the system throughout, connected to the D10 via USB and to the S6 via RJ45 at the same time. This arrangement allowed me to alternate between the two playback scenarios I originally had in mind. In the first, music travelled over an optical network link from the S6 to the D10’s internal streamer. In the second, the D10 switched to its USB input, while the Statement Next-Gen resumed its familiar role as both streamer and server. Consequently, the comparison extended beyond an optical SFP link versus a conventional Ethernet connection. It also involved two distinct playback architectures: in the former scenario the Statement functioned solely as a NAS, whereas in the latter it acted as a complete streamer/server feeding the D10 over USB.Needless to say, this wasn’t a textbook A/B comparison of SFP versus RJ45 alone. Additional variables inevitably came into play. Rather than viewing that as a drawback, however, I treated it as an opportunity to find out whether the practical benefits of a fully optical network path were compelling enough to outweigh the architectural differences between the two digital front ends. In other words, I wasn’t trying to isolate a single variable in laboratory fashion, but to determine which approach ultimately proved more convincing in actual listening. If one scenario involved the D10’s internal streamer fed over an optical SFP link, while the other relied on conventional RJ45 upstream, USB downstream and, consequently, a standalone streamer that costs considerably more than the D10 itself, it was only fair to ask what, if anything, such a comparison could actually tell me. With so many variables at play, was there anything meaningful to be learned?

As it turned out, quite a bit. For starters, the exercise offered a rare opportunity to gauge just how much the Innuos Statement Next-Gen actually contributed to the result. In high-end audio, outright preference is only part of the equation. Equally important is the scale of improvement relative to the investment required. Spending substantially more should, ideally, yield a proportionally greater reward. If the D10’s own sanely priced streaming platform, fronted by the COS S6 and its companion LPS1, could approach the performance of the Statement operating as a dedicated streamer, that alone would constitute a significant finding. Viewed from that perspective, the comparison became less about isolating SFP as a single variable and more about evaluating the effectiveness of an entirely different digital playback philosophy. The real question wasn’t whether optical networking sounded marginally better than conventional Ethernet. It was whether an SFP-enabled playback chain allowed the D10 to perform so convincingly that the case for investing in an external high-end streamer became substantially less compelling.Any review involving network audio tends to come with its fair share of logistical headaches. Given the setup outlined above, I fully expected this one to be no different. Two streamers in play, switching the D10 back and forth between its network and USB inputs, and juggling two separate control apps sounded like a recipe for frustration rather than an efficient evaluation process. To my great surprise, it wasn’t. In fact, the entire back-and-forth stage proved remarkably seamless. The credit goes to Innuos’ excellent Sense app, which automatically recognized the D10 as a UPnP renderer and listed it alongside the Statement Next-Gen. That meant I could carry out the entire evaluation without ever leaving the listening chair. After selecting the appropriate digital input on the D10 via its hefty aluminium RC wand, with the unit’s volume control bypassed, I simply chose either the Statement or the D10 as the playback destination within Sense. A couple of taps and the entire playback chain reconfigured itself accordingly. No cable swapping, no reconnecting devices, no software gymnastics. Just few button presses and music. Frankly, it felt almost suspiciously civilized for a network-audio comparison.

In my original Statement Next-Gen review I remarked how this platform sounded exceptionally direct, dynamically gifted and rhythmically assured. What struck me most wasn’t merely its ability to resolve minute amounts of information, but the sheer conviction with which it delivered every transient. Notes started and stopped with remarkable punctuality, dynamic swings happened instantaneously and without hesitation, while rhythmic structures locked into place with an almost metronomic precision. Whether tasked with explosive orchestral climaxes, intricate percussion work or rapid-fire bass lines, the Statement always felt eager, alert and fully in control, never once giving the impression of lagging behind the music it reproduced.Perhaps even more impressively, that speed and incisiveness never translated into sterility or fatigue. The Statement Next-Gen paired its lightning-fast reflexes with excellent tonal saturation, generous body and an effortless sense of scale, which prevented its highly articulate presentation from becoming analytical or mechanical. It was a source capable of delivering tremendous impact, startling transient contrast and exceptional rhythmic drive while remaining composed, fluid and naturally musical. That combination of snap, precision and dynamic authority ultimately became one of its defining qualities and the benchmark against which I instinctively evaluated every streamer that followed. I should also mention that I’m unapologetically biased towards a very specific sonic ideal. Wide dynamic swings, explosive transient response, immediacy, sheer impact, and an unwavering sense of rhythmic precision are the qualities I value above all else. My entire reference system has been assembled with exactly those priorities in mind, courtesy of the Innuos Statement Next-Gen as much as the Horizon360 DAC and my sound|kaos monitors. Guilty as charged.

That preference also explains why the Statement has remained a permanent fixture in my system. Over the years I’ve auditioned a fair share of digital front ends, yet none managed to better it in the areas I personally consider most important. Some excelled in refinement, others in tonal beauty or sheer resolving power, but when it came to dynamic authority, snap, punctuality and the ability to propel music forward with infectious conviction, the Innuos continued to thrive. To the best of my knowledge, the only platform likely to surpass it outright is Innuos’ own flagship NAZARÉ. That, however, belongs to an entirely different financial league and is a story for another time. To cut through the suspense, the D10 connected to the Statement via USB still held a clear advantage in all the sporty disciplines. It was the quicker, snappier and more dynamically expressive performer, and I can’t say that came as a surprise. After all, the Statement Next-Gen is what it is. Yet stopping the comparison there would do the D10 a considerable disservice, because the SFP-fed route had a number of aces of its own, and they announced themselves so clearly that mistaking them for anything else simply wasn’t an option.Used as both streamer and DAC, with the S6 and its companion LPS1 feeding it over an optical SFP link, the D10 painted a noticeably different picture. The presentation was more anchored, denser, richer in color and undeniably more invested in the artistic rather than athletic aspects of music reproduction, although even that doesn’t fully describe what I heard. Above all else, the result was astonishingly fluid, moist, organic, fruity, sensual and positively overflowing with harmonic richness. Resolution was by no means sacrificed either. Quite the contrary. The tiniest ambient cues, faint reverberation trails and that familiar plankton of low-level information remained abundantly present, yet they were never spotlighted or pushed to the foreground. Instead, they emerged with complete naturalness, almost as if retrieving such subtle nuances demanded no effort whatsoever. That effortless quality atop fluidity and grand tone, more than any individual sonic trait, defined the D10’s SFP-fed performance in my system.

Come to think of it, the D10 used over SFP consistently reminded me of the qualities that well-recorded DSD material so often brings to the table. There was the same sense of liquidity, continuity, harmonic completeness and unmistakably analog charm that makes one stop dissecting individual sounds and instead surrender to the musical event as a whole. In that context, the Statement Next-Gen felt more akin to exemplary high-resolution PCM playback. Its presentation favored tauter suspension, greater transient illumination, explicitness, intensity, and crisper contours, while placing less emphasis on their internal fill. Neither approach struck me as inherently superior. They simply catered to different aesthetic priorities. If anything, they left me wishing I could somehow combine the best qualities of both into a single digital front end.What proved most surprising, however, was how compelling the SFP-fed D10 remained over extended listening sessions. The more time I spent with it, the stronger the impression became that this wasn’t merely a different flavor of digital playback, but a genuinely accomplished one with a distinct identity and undeniable appeal. To the point where I’m willing to make an educated guess. Had I been able to compare the D10’s SFP streaming module directly against its RJ45 counterpart under otherwise identical conditions, I strongly suspect the overall tonal character would’ve remained much the same. The optical version, however, would’ve likely retained that organic, harmonically generous presentation while narrowing much of the remaining gap in terms of speed, transient snap and dynamic authority. I can’t present that as a conclusion drawn from direct experience, only as an educated inference informed by everything I heard throughout this evaluation and my understanding of what SFP brings to the table.

There’s another important aspect to this story. Had my reference streamer not been as dynamically gifted as the Innuos Statement Next-Gen, I suspect I would’ve preferred the D10 fed over SFP outright. As things stand, I don’t. As much as I admire what optical networking brought to COS Engineering’s all-in-one device, and as brilliantly as the S6 and LPS1 elevated its streaming platform, the Statement still had that extra measure of bite, immediacy and explosive dynamic expression that I personally value above all else. Given the priorities I outlined earlier, I would’ve genuinely missed those qualities.

At the same time, I can’t help but wonder how this comparison would’ve unfolded against a different streamer. Had my reference source been less accomplished in those specific performance-oriented traits, the outcome might well have easily tipped in the D10’s favor. That thought alone speaks volumes about just how convincing the SFP-fed COS setup proved to be. It didn’t merely narrow the gap to the finest streamer I’ve had the pleasure of using; in several areas it presented a thoroughly compelling alternative with a character all its own. Long story short, in this hobby SFP really is a thing to pay attention to. Lastly, credit where it’s due. The folks at COS Engineering deserve praise for embracing SFP-ready infrastructure across their digital products. Having now spent considerable time with a fully optical playback chain, I’m convinced this approach offers something genuinely special. I also believe far more audiophiles should be aware that such an option exists. If this revisit of the S6 and its SFP capabilities encourages even a handful of readers to look beyond conventional RJ45 networking, then writing this follow-up will have been well worth the effort.

Associated Equipment:

Retail prices of reviewed components in EU (incl. VAT):

  • COS Engineering S6: €1’400
  • COS Engineering LPS1: €1’400
  • COS Engineering DCC2: €300/1

 

Manufacturer: COS Engineering

 

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