That's great news on the distortion fix!
Do you know what version of firmware has the fix (1.50 perhaps)? I'm running 1.41. I can't find any release notes on 1.50.
Background:
I got the NAD M51 as an upgrade over an Emotiva XDA-1 DAC and there was a definite improvement so I kept it and sold off the Emotiva. Unfortunately, I found that in the "Fixed" volume setting driving my Pass Labs XP-20 pre-amp over balanced cables, it was distorting BADLY.
If I backed off the gain, the distortion would diminish to inaudible levels. I did not think that was quite right and was worried that the NAD was defective. Much back and forth with NAD and Pass Labs ensued. End of the day, I had to "settle" for backing off the gain to -6 (distortion was gone at -5, but I took it down to -6 to avoid distortion which might have cropped up on hotter sources).
Pass Labs said the XP-20 should not be overdriven based on NAD's published specs for the M51 but it obviously was (either that or the M51 was being overdriven internally before sending signal out). Running other devices as inputs to the Pass Labs XP-20 preamp over balanced cables caused no distortion. This included the Emotiva XDA-1, a Musical Fidelity M1 DAC, and a Benchmark DAC1.
A discovery:
If you are in the market for a $2,000 DAC, you owe it to yourself to hear the Chord QuteHD (now QuteEX). It's a significant improvement sound-wise over the M51. (I saw a posting in this thread where somebody preferred the _feel_ of the NAD over a Marantz -- if heft is important to you then the QuteHD is NOT your thing). Very straightforward A/B testing resulted in immediately obvious, repeatable improvements that everybody who's heard this A/B test has picked up immediately. The listeners are not told what device they were listening to. But when the signal path was through the QuteHD, the sound was "clearer", "less veiled", "more present in the room", and "less scratchy and harsh"... I've been through a lot of DACs and never heard better. QuteHD leaves the M51 in the dust. I suspect the current QuteEX is either the same or better.
I've kept both the M51 and the QuteHD. I use the NAD for exploring music or streaming content via a Google Chromecast Dongle because it outputs over HDMI only. I also kept the NAD because I use a Pioneer transport to play formats that require HDMI out (SACD). The NAD has 2xHDMI in and that's the only reason I'm keeping it. Everything else is played through the QuteHD.