Man you're lucky. I thought there were a few who went back and had that scramble the M51 up bad. I never heard 1.39 as I started at 1.41 before going to 1.50. Personally I certainly couldn't hear a difference between those firmware revisions. I'm assuming a great deal of the revisions is to solve smaller niche problems that arise as NAD gathers users feedback more than sound quality, but certainly I don't know for sure.
The documentation for 1.39 states that it is only recommended for units prior to s/n H25M51XXXXX. I've had my M51 for some years ( s/n H29M51XXXXX) and it came with 1.39 installed, so I knew (hoped) I wouldn't have trouble reverting back from 1.50.
I haven't heard the other firmware updates. I stayed on 1.39 for a long time as I was very content with the SQ and reports online that others also preferred 1.39 gave me no cause to upgrade. Stereophile did quote NAD as stating that there is no tweak to the M51's signature in firmware post 1.39 - other than a -1dB adjustment.
Initially, I did not regret the upgrade. In terms of voicing, nothing really jumped out as drastically different to what I was used to. After some time with 1.50 though, I noticed a slight lack of energy. The M51 seemed more polite and less engaging, and in the background, it became obvious that the lower regions were a touch less dynamic, affecting the presence and impact of bass and percussion instruments.
Increasing the M51's volume +1dB did not seem re-balance 1.50's signature to what I was used to hearing from 1.39, as that adjustment also raised the volume of the upper frequencies. This initially gave the impression that the presentation was perhaps a touch more neutral in this firmware, with a little more detail in the upper registers, but after reverting back to 1.39 it is evident that the M51 is not blunted in those regions.
After reverting back to 1.39 I played the same albums that I had just listened to and had found to be a touch flat and somewhat polite on 1.50. With my amp's stepped volume control set to the same setting and the M51's volume set -1dB less than where I had it - to accommodate for the documented difference between firmware revisions - I once again heard the same musically engaging character I was used to hearing from the M51.
I went from 1.39 (the original firmware on my unit) straight to 1.50. I managed to revert straight back. I've had no other firmware installed on this unit.
edit: Firmware 1.39 is found in this post - http://www.head-fi.org/t/602771/nad-m51-direct-digital-dac-impressions/660#post_9140977. Proceed at your own risk.
Edited by olor1n - Yesterday at 6:58 pm