I am posting a few comments minus sound comments. I used Romani's review format to type these comments, thanks so much Romani!!! If there are any questions about anything I opinions I have on these items, please don't hesitate to ask.
Sources: Fiio X5, PONO player, Moto X streaming Tidal,Lossless music formats, mp3's.
Usage so far: I have had about 40-50 hours of listening to these.
First impressions: Great! mm400 really rock!
Packaging: mm400 not much to it, very simple; M2 simple, box has foam insert inside foam is a protective case, inside the case is a black bag with M2 inside - Win M2(only slight better presentation)
Size: Portable - Both headphones, No case for MM400 only black felt type bag, M2 has a case and a black bag - Win M2(the case)
Portability: Both are portable I feel strange with the Denon outside the home walking or otherwise because they look like a top shelf equipment,wood,chrome and design, M2 has more "cool" look a little more utilitarian. Both cans fold. The case for the M2 is a little big but could fit in a back pack - Win M2(the case; looks)
How they look on: Neither headphone is too large. The headband on the mm400 is not as bad as the HP50 but it does not conform to the line of the head, it sticks out a bit. The M2 looks "cooler" on the head. I wear both outside but would prefer the M2 - Win M2
Build quality: mm400 is beautiful!!! Wood covered cups, metal swivels and sliders and chrome inlays on the wood cups. The swivels and ability to move the cups on your ears, well built , well designed. The M2 is engineered; functional. I have not yet encountered the "noise" from the hinge point in the mm400 so I can't count that. The earpads on the M2 are real leather so soft almost a gooey feel, not creepy gooey luscious gooey. I wish the earcups from the M2 were on the mm400 - Win mm400
Value: Not fair...the M2 is less expensive and received a great deal on them. The retail is normally 350.00 for the M2 and 400.00 for the mm400.
Comfort: Both headphones are very comfortable. I like the comfort of the mm400 headband padding and wonder why the M2 puts little to less than little padding on the headband. The earcups on both are very comfortable, but as stated prefer the earpads on the M2. Initially, until you get the headphone adjusted the way you want the mm400 is more comfortable. - Win mm400
Leakage/Isolation: The M2 isolate better in the "world" but with music playing you can't hear much from the outside world with either headphone. The M2 isolate well. - Win M2(but who cares, with music playing they are equal)
That is everything up to sound...obviously some of the above is only my opinion, but this is all without taking sound into account. Cool factor, looks, etc... are a matter of taste, many of these things are a matter of taste.
I will add sound to this ongoing....