Do Streamer only devices really impact sound quality?

For many devices recently on the market, the streamer is integrated with a DAC and possibly preamp capabilities (volume control, source selection, or both). So chances are, you won't be hearing the "streamer itself" in isolation from one or more of those other functions. Furthermore, streamer A and streamer B may be delivering different combinations of software effects (DSP) or source content, which all color the output. FWIW, Stereophile does not have a separate category for "streamers" in its annual list of recommended components. They have a "digital processors" category, which mostly covers DACs (many of which have integrated streaming or preamp capabilities, equalization, DSP, etc.) 

So, while In theory "everything is important", I doubt that anyone here has done controlled, volume-leveled A-B comparisons among multiple streamers in a way that focuses solely on the streaming functions, per se, of all the compared devices.  I'd venture to say that the impact on SQ of the streaming capability, per se, probably is somewhere between negligible and unobservable compared to the impact of the DAC and especially the preamp effects.  But I don't really know for sure, and would welcome anyone to point out a good comparison of multiple streamers, one based on some credible combination of metrics and expert observation. 

I did a quick search for Darko's streamer reviews.  Not sure I found the one @soix cited.  Anyway, Darko's highest praise seems to go to the Logitech Squeezebox Touch.  You can get one of those on eBay for less than $200 (US).  I wonder if anyone here has heard a significant sound quality improvement in going from the SBT to, say, the Auralic Aries G2.1 ($5699).  Not necessarily 30X better ... but at least noticeable to the average listener.

Not sure what to make of that title.  I suppose Darko might be saying that the SBT was a relatively affordable engineering marvel, not that it's the best-sounding such device ever made, regardless of price. The same might be said of the latest Bluesound Node. But the Node is not only a streamer/file-player, it's also a DAC.  It can be used for volume-control, too. I've seen reports that it's quite good when used only as a streamer (feeding digital output to a downstream DAC).  If that is our baseline, how much SQ improvement can one expect by switching to a much more expensive stand-alone streamer (such as the Auralic Aries G2.1)?

Individuals use usb because it has best SQ vs other inputs/outputs available to them. I've observed many trying coax, AES/EBU, I2S inputs on dac, finding them inferior and returning to usb.

I don't know that the channel itself necessarily makes that much difference, to my ears. What makes a SQ difference, to me, is the various DSP options I can get (or cannot get) via one channel or another.  I need a USB connection to a Macintosh computer in order to run BAACH4Mac (a spatial audio processor). I need a USB connection to a Mac, or to a dedicated music server (with an ultraRendu in the middle), to run HQPlayer (which offers many filtering and upsampling options).  If I want multi-room playback, I cannot run either of those processes, so in that case I use ethernet (or, in the past, coax or RCA analog).  If I want to play TV sound through my HiFi, I need Toslink (because my system does not have a complete HDMI ARC path from HiFi to TV and back).

IOW, functional requirements  (as well as SQ) dictate the choice of streaming signal path, for me.  I've had at least 4 different streaming pathways in my system. These include the Bluesound Node 2i (which FWIW has one of the worst ASR measurements for a device of its kind) and also a Matrix Element X (which FWIW has one of the best ASR measurements for a device of its kind).  I haven't noticed enough SQ difference among those paths, per se, to override the functional differences.  And without going to a ~$30K T+A streamer/DAC (one  that supports  the NAA protocol), I doubt any of them delivers enough SQ improvement to justify doing without HQ Player (or BAACH4Mac).  Which I'd like to do, just to simplify the chain. But I'm skeptical that a change of streamers alone can make my soundstage sound about 50% wider, as BACCH4Mac can for some content. 

I wonder how many people have compared I2S with USB in a well-controlled A/B test.  I2S still isn't all that widely available.  My streaming DAC does have an I2S input but I've never used it.  In my system, the most likely path into that IIS port would be from my Mac's, uh, USB port, via a converter box such as the Sonore ultraDigital or the Matrix X-SPDIF 2.  I've been slightly curious to see if that would produce a noticeable improvement, but don't really want to add another little converter box into an already cluttered system. Besides, the SQ already is pretty darn good.  But I do wish my music server (the SGC sonicTransporter) had an IIS output port. My DAC with IIS input can support 16-32 bit PCM, while its USB pipeline supports only 16-24 bit.  Perhaps more importantly, IIS might eliminate the annoying USB handshake problems I sometimes experience.

IIS from the Matrix X-SPDIF seems to be popular among users of the PS Audio Direct Stream DAC. I wonder if this popularity doesn't stem from limitations of PS Audio's USB implementations, or from problems with the Direct Stream's network bridge card. Point is, again, that if we're going to compare USB to IIS, I think we need to specify the context of how USB is being implemented in the system we're comparing.