I've got it hooked up through a chord co. optichord cable.
Did not try coax since my pc doesn't have one.
I wouldn't bother with usb with the rega dac, since they don't seem to support higher bitrates/sampling frequencies or something of that nature, and I have optical outs.
More important than the cable, IMHO, is using a bit perfect player (eg. FooBar2000), and using wasapi instead of the windows mixer. The windows mixer will muck with the files.