HI - I’m running roonserver on a QNAP TS 251+ and have both ethernet and USB out working perfectly.
My question is; does roon work with any USB DAC that supports USB Audio Class 2.0 without installing specific drivers?
I’m currently using a Devialet amp which Im thinking for changing soon and want to use USB out on the QNAP to a USB DAC.
Probably pretty noisy, electrically speaking.
Most audio devices will be seen by linux (and QNAP). A few require certain kernel options (which is not doable on the qnap) or do not play with linux at all. If you have the option to test it, this would be the best route to go. Otherwise I’d search the web and the manufacturer’s website regarding linux compatibility.
A test would also give you the opportunity to judge for yourself if the device suits your wishes. 
Only if not async and galvanically isolated.
I Use my QNAP-TS 470 Pro as a Roon Core and server.Connected it to dCS U-clock & Puccini Dac directly.It has a USB Clas 2.0 input.
Usb cable is a pure silver Audioquest Diamond cable.
Exellent sound, no issues.Works perfectly.