Roon states best practice as using Ethernet for best sound quality. I, however, have found that direct connection from Nucleus+ via USB to DAC is best in my main system. I have tried quite a few different scenarios and always come back to connect DAC direct via USB to Nucleus+.
I have tried mainly:
N+—> network —> Sonore microRendu —> then to PS Audio Stellar GCD —> PS Audio M700s to KEF LS50s
Same as #1 except DAC is Chord Qutest between Rendu and then to GCD via analog
N+ —> iFi jitterbug —> Curious USB cable —> PS Audio Stellar GCD …
N+ —> iFi jitterbug —> Curious USB cable —> Chord Qutest —> PS Audio Stellar GCD
I prefer the USB connections over the Rendu regardless of which DAC. Better soundstage, macro dynamics, drive, separation, etc. And I prefer the Qutest over the Stellar GCD’s own DAC, which itself is very good, but not quite at the level of the Chord. Just bought a second Qutest actually to use on a secondary system I liked it so much.
In any event, although Roon does not encourage USB connection as the 1st choice over Ethernet, I find in my system USB is better. Maybe this is the limitations of the Rendu. Perhaps a better streamer would provide better results, but I’m extremely happy going direct from USB to DAC. Incidentally on secondary system I am using a Rendu connected to another Qutest and getting near qual results to main system which is direct via USB. That system of course has different gear, so it goes to show how each component in your system matters and experimentation is the only way to know what works best in your system and for your ears. (The secondary system has the Rendu/Qutest going to Emotiva USP-1 preamp and Emotiva XPA-3 Gen2 feeding Elac Unifi UB5s. The old your mileage may vary phrase comes to mind…a lot depends on your system components and how they match up and work together or not.)
Incidentally, I am generally upscaling from Redbook 44.1 to 768 PCM. I find that DSD while occasionally providing better soundstage/imaging removes some of the drive and bass oomph/attack. Also DSD occasionally has some flaky behavior with pops/clicks, dropouts, or just is not available when it seems it should be. (Not available via direct USB connection but is available when going to Rendu first.)