I owned the M3 and own the MA3. I think the M3 is a fine DAC but much prefer the MA3. I didn't own them at the same time, but will say I find the MA3 to have a much more organic/analog sound but still with a good level of detail and zero fatigue. I remember the M3 as having good detail retrieval and excellent dynamics, and not fatiguing or analytical, but I personally found it slightly on the 'thin' or 'digital' side. I just couldn't get into the music much more than I did when I tried also a very fine sounding but MUCH cheaper RME DAC. I replaced the M3 with a Mojo Audio R2R and as a reference point that was much more the sound I was looking for.
I think the M3 had a more modern sound (i.e. better detail retrieval and blacker background) than the Mojo but I happily gave that up for the sound the Mojo provided.
Then I heard the MA3 and think it is a great blend of the strengths of the other two. It reminds me of my Mojo's very musical sound but with another level of detail retrieval and soundstage, without adding ANY hardness, leanness, or fatigue.
Cheers