I have had an M51 for about two months now and find it a fantastic DAC. But when used as a DAC/pre, it really shines. The volume control is second to none I have ever used. The DAC section I have found to be my favorite so far. I actually sold my Dodd Battery Preamp and Metrum Octave once I listened to the NAD. And I liked the Octave better than my (now sold) Weiss DAC2 and Berkeley Alpha (1st version). Having the HDMI inputs is a great addition.
The USB input is very good, but significantly bettered by my Empirical Off Ramp 4 into the coax of the M51. As a matter of fact, my current digital source (Amarra/MBP/OR4/M51) sounds so good, I sold off all of my vinyl equipment. None of my prior DAC/pre combinations ever reached this level.