I'm almost positive that the bass opens up and significantly improves as the PM-3 is used. Most people in this thread claim brain burn in or just getting used to them, but I disagree since I've just been going back and forth between them and the M2 for hours on end. When I first pulled it out of the box, listening to hip hop was so sterile and boring compared to the M2 which just seemed more lively and energetic. Yes, it's definitely a more V shaped headphone, but the gap between the two in that type of a comparison was huge. After listening to the PM-3s most of the weekend and just using pink noise when I wasn't listening, the PM-3's bass has closed that gap significantly. It's still a more balanced headphone overall with no real big anomalies. This just made the headphone quite a bit more engaging and fun. So color me impressed. So impressed that the M2 is on it's way out of my house. Being more of a hip hop guy, I need that bass...not bloated mid bass humps like the M100s (my previous favorite headphone)...something that is more balanced that can hit pretty hard when pushed.
All these claims about the PM-3s being so sensitive, you can run em off any device is just a little over exaggeration. The M2s go quite a bit louder than the PM-3s...and w/ poorly recorded music, I need to turn up my GS6 almost all the way. It's not like that with regular music ripped recently, but older stuff is rough. The M2 is just a louder and more sensitive headphone that can be run out of portable devices better than the PM-3. Oh, and they've got a proper Android cable w/ the volume up and down as well, unlike the PM-3s one button Android cable. Sennheiser def pulls ahead in that regard. Either way, I normally use my IEMs w/ my phone, so it's not a huge deal, especially the remotes, but it's just nice to know the option is there.
You are correct. The PM-3 can do with a bit of amp'ing. The M2 is much more fun with all types of music. I had a few ah hah moments with the PM-3 and when I was at home listening or with a portable amp the PM-3 sounded great. I sold the PM-3 to a fellow head-fier because I just didn't need another hp to listen to in a perfect environment. That said I am excited to hear the Denon AH-MM400 when it arrives today.