I was listening over S/PDIF these days and I thought it sounded just a bit better compared to USB - a bit punchier. I asked my girlfriend to help me with a blind test - it was a simple matter of her choosing different outputs in the player while DAC was connected to PC both ways... and I chose S/PDIF every time as being a bit livelier and punchier. So I'm using it that way now.
Difference is not really that big and important for the real life listening experience and I wouldn't care if I had to use USB for any reason. It's just interesting to notice that it is audible which I didn't expect it to be, and no - you don't have to be a bat to hear it.
Anyways, it sounds so good that I'm pretty sure it will stay with me for quite some time, so I ordered
this nice case with thick front plate and I'll drill and file holes myself, hoping I can make it look half decent.