I have no experience with the nad dac but have used both the usb iPower and the usb-spif converter, the iLink, to my dac. The iPower splits the usb power from the audio data with a double-headed usb cable. I find both essential in improving sound quality from the pc.
NADM51 whit Ifi usb purifier or ifi usb converter
I have to plug my m51 in my computer, when i plug it directly in usb the result is not very good compare to my cd transport. The ifi audio compagny offer usb purifier and usb to spdif connection for the same price. I would like to know before making my purchase which one could give me the best result.