Both, I believe. Had ground-loop from my setup involving a very cheap passive mixer and a an asortment of other cheap crappy usb dacs, and there was definitely some EMI involvement as you could hear it change as I moved my cables around the tangled mess of other high speed data cabling.

Don't have the equipment to test or diagnose, but empirically, having moved to using balanced cabling from point of analog conversion all the way to amp input, there's no sounds coming out of my headphones that shouldn't be.