Hope you get the usb issue sorted Gary. I don't want to influence your findings but on my system there is an audible difference between the M51's usb (integer mode from MBP) and optical inputs.
Not sure what the issue could be for you. The M51's been set and forget for me from day 1. I've never had a Windows system connected to it though.
If you have an SPDIF cable around you could use the Concerno as USB converter for the NAD. It would be a not quite accurate view of the NAD, but likely better than stock and perhaps a more accurate view than toslink.