My 2c on the M2 and PM-3. I think the following 2 tracks best illustrate for me the differences in sound signatures between the two.
First off is Bubbles from Yosi Horikawa (https://soundcloud.com/yosi-horikawa/bubbles). This song was made by sampling the sounds of bouncing ping pong and other balls. I think this song highlights PM-3's strengths in instrument separation and imaging. The sound of each ball dropping (out of hundreds) is distinct from the others and has impact, like you can actually feel the balls dropping near you. The M2 is not quite as resolving and lacks some of the impact of the PM-3 but the overall sound is more cohesive. One the M2 the sound of the balls dropping feel more like they're an integral part of the music whereas on the PM-3 they sometimes feel detached from the music and separate.
Next up is Tamacun by Rodrigo Y Gabriela (https://www.youtube.com/watch?v=vULsGEm4x4s) which is an incredibly fast paced acoustic guitar duet. On the M2, this song has a fairly wide and airy sound where the music definitely feels like its coming from outside your head, especially at the 1:30 mark. On the PM-3 the soundstage on this song is pretty narrow to the point of feeling congested with all of the music firmly originating from in between the headphones. That said, PM-3 picks up an amazing amount of detail and has no trouble keeping up with the pace of the individual notes. The M2 is a bit slower in this regard so the sounds blend into each other somewhat. However, I think this slight blending on the M2 brings the music together. There are times on the PM-3 where it did too good a job of separating the sounds and it felt like I was listening to 3 distinct components (rhythm, percussion and melody as separate discrete parts) rather than a unified whole.
Wish I could have linked to a higher fidelity version of Tamacun. I think the effects that I'm describing are a bit harder to discern due to the crappy youtube version.
In conclusion, I'm impressed by both headphones. However, I prefer to have headphones that help me kick back and relax so my preference goes toward the M2 which is more musical although it falls a little behind the PM-3 on technical ability. If my headphone enjoyment came from critical listening and being able to pick out details from the music, my hat would go off to the PM-3.