I use optical from my desktop, and the sound from the Mojo is very clean and precise.
If i use usb from my desktop, I can occasionally hear short bursts of low level electrical noise (a bit like the sound of a record stylus being dragged 1mm across a record) during very quiet passages - however just as Rob Watts predicts in his posts, this electrical noise makes the music sound slightly brighter.
Mostly I stick with optical, but occasionally I like to try usb for some albums.
You will only find out if you prefer optical by trying it, and all you need is a $5 cable - so it will not be an expensive experiment.
I don't hear any form of electrical noise during quiet piano passages and such. It's very, very dark.
My setup is:
Asmedia USB 3.0 port from a Z97X chipset with i7 cpu and high quality EVGA PSU --> Supra USB Cable --> IFi Nano iusb3.0 -> QED Reference 0.30m usb cable --> Fidelio X2.
Only thing I hear is random pops and clicks when listening to music via Directsound (not aiso or wasapi) while streaming music at 44.1 khz. I think if windows upsamples to 96khz 24bit, then the pops disappear
Other than that, it's very musical with no hint of brightness I can percieve at least.
I'd still like an answer from someone qualified within electrical engineering if it can cause audio problems to have an ungrounded PC as the music player. For what I can read it's a plus because of no ground loop/common ground-issues, but I'm a newb, so what do I know.
Fidelify is great to get bit-perfect music through spotify btw. you have to open spotify first and fidelify first together. Then close spotify down, and now u can search in fidelify after all the music ya want to hear, and it sends it through AISO chord drivers directly to your mojo without windows interfering