I can add a few words about it after visiting my local denon dealer.
I was very sceptical about smaller diaphragm diameter comparing to d5000/d7000 but decided to give it a try anyway at my local denon dealer and I am still in shock how good it sounded. Tested mainly on classical most demanding tracks with high dynamics, hard to resolve tonality and upper harmonics (piano, cello, violin, large orchestra - amazing resolution, nothing is lost during complex passages, clarity and treble are freaking out of this world and really natural). When playing instrument solo there are a lot of small details that many other headphones can only dream about. Then I played some jazz tracks. Oh my lord ... I loved the speed, transient response, details, thunderous attack and saturation of instruments. Saxophone played lifelike, piano had proper weight and reverb, upper midrange was perfect (something i didnt like at all in d5000/d7000, m400 upper midrange is something like in high models of hifimans which I love - has that spark and bite, is a bit raw and really adds some realism) resolution was outstanding - no matter how many instruments very playing at the same time. I am thinking about buying this headphone. Maybe I will get it for 1 day test and check it out in my home setup because I tried it only from my samsung galaxy output with many especially prepared tracks for testing (LOL !) and it played so amazing.
m400 is extremely comfortable in my opinion. Despite not too big diaphragm my ears got covered completely by it. It completely disappeared from my head after a few minutes of listening.
Tried m300 also but it is in completely different league, not that clarity and resolution. It easily loses control too with bass comparing to m400 when bass is perfectly controlled even from portable device driving it. Also tried some bose stuff being really curious, my lord ... not gonna comment it.