Apologies for the long post. Writing a shorter one would have taken longer...
I've spent a lot of time over the past couple of months playing around with my new music server/streamer computer, new DAC, my old Squeezebox Touch, a variety of digital output methods (USB, coax, optical), Roon, JRiver, Squeeze Server, and a variety of audio file formats. I wrote briefly about this in another post earlier this year. From the experience I've reached a number of conclusions about computer audio that are relevant to this thread and for anyone spending far too much time worrying about this stuff. This may be old news to many of you but it was fun playing around first-hand.
The goal was to replace my Macbook Air with a headless computer but it got a bit bigger as I went along. I wound up replacing my built-in DAC on my McIntosh C48 with a Chord Qutest. The PC I built uses an Asus thin ITX board with a Celeron G1840 (2.8 GHz dual core 53w TDP) set up in a fan-less chassis using a 6TB USB drive for music storage. For grins I tossed in a Matrix Audio Element H USB 3.0 pcie card so that I could isolate USB output to the DAC from the rest of my computer. The board can be powered by an external power supply. I installed Windows 10 on the PC and for playback I installed JRiver 26, Roon, and Logitech Media Server. I really wanted to compare Roon and Jriver. Logitech came along when I was playing around with my old Squeezebox Touch.
Long story short, I can't hear any difference between USB output between the Matrix Audio board and the native USB 3.0 ports on Asus motherboard when playing the same music track. I also can't hear any difference between USB cables between Monster Cable, the unbranded USB 2.0 cable included in the Qutest, and a Blue Jeans USB 2.0 cable. The Monster cable was 6 feet, Blue Jeans 3.9 feet, and generic cable was 4 feet.
While I was at it I also bought a coax to BNC cable from Blue Jeans so that I could compare optical and coax SPDIF output from my old Squeezebox touch. I also installed a third party app on the Squeezebox that lets me use the USB port as an async USB output to my DAC. Now I have a set up that lets me compare coax, optical, and async USB into the Chord DAC using two different platforms although the PC is limited to only async USB.
Playing the same track over async USB, coax, and optical sounds the same to me through the Chord Qutest. There were times when I felt that coax was sounding better than optical on extended casual listening only to find out that had optical selected. The reverse sometimes happened too. To be clear, all delivery paths sound fantastic and with high def audio files each path is equally excellent.
With async USB I can hear no difference between USB from any of the PC USB ports, the Squeezebox touch (using the third party digital output app), or the legacy Macbook Air. Each sounds super good. I also can't hear a difference with the fancy Matrix Audio USB hub even if I'm powering from battery or any difference if I power the Qutest DAC from battery. I'm superstitious about the consistency of async USB so that's what I use for normal listening.
I initially noticed differences between JRiver, Roon, and Logitech MS but those differences went away once I made sure that I normalized how audio files were processed. Each app has its own view on how it wants to equalize sound levels. It not surprising that Roon sounded slightly better because it tended to playback with slightly higher sound levels. Once I turned off sound level equalization there was no longer any difference between playback software.
At the end of all this experimenting and playing around I feel pretty strongly that if you have a well engineered DAC then async USB will provide a consistent experience from a pretty wide variety of streaming devices. You'll also get terrific, if not the same, results using coax and optical.
I don't hear any reason to segment the music server computer into separate server and streaming end points. The PC and the Squeezebox touch sound the same playing the same tracks. There's also no audible penalties for doing so if that's a more convenient configuration.
There's also no reason to go big with a music server. Mine is built around 4th generation Celeron processor that cost me $40. My entire PC build was less than $700 and $250 of that was wasted on the Matrix Audio USB card. Another $200 went into the chassis but that made the PC build really easy to execute with the included heat sink and pipes so that seems well spent.
If I were to do it all over again I would either use a mid-range Intel NUC or build again on an ITX board with a fan-less chassis for the music server. I would not waste any money on hi-fi audio specific USB cards. I think the Chord Qutest sounds fantastic regardless of how it's fed. I also wouldn't waste any money on expensive music streamers. RasPi solutions are about $50 and if I can't hear a difference between the various async USB connections I tried then I'm skeptical that spending more money will yield a benefit. Same goes for fancy power suppliers including batteries.
Where I did run into problems was with playing back some high resolution DSF files. JRiver on my Macbook Air can't seem to cope with those files and the Logitech MS / Squeezebox Touch combo had some problems with these files too. With Roon and JRiver on the new PC there are no problems. Maybe I'll replace the Squeezebox touch with a RasPi and touch screen as a future project.
I'm happy to host anyone that wants to give a listen to the different delivery paths. I hope folks find this useful. At the end all this experimenting I came across the blog post that agrees with what my experience: http://archimago.blogspot.com/2017/03/musings-computer-audio-part-ii-basics.html