Yesterday I had the MMX 300 2013 Edition on my head for 2 hours L4D2 and half an hour MoH: WF and it's the most balanced "DT770" I ever listened to (I actually listened to all of them) the smoothest highs and a non bloated or overblown bass. Actually no overpowering bass at ALL so quite ready for competitive play.
Before the DT770 LE had the smoothest highs of all (to me) yet still with a hint of sibilance at times and the bass could sometimes be too much, still less than all other models. The MMX 300 is even better in that case as in only hitting hard when it needs to, generally more balanced along the spectrum. "Marina and the Diamonds - Shampain" is one of those tracks that exposes sibilance very very well and the MMX 300 actually passed it 100% to my surprise. The DT770 LE did not that well, the T90, Pro Ultrasones failed horribly. It has the biggest soundstage (more in width though) of all DT770s but also requires more power than the DT770 LE, TH-900.
When I checked with Beyer they mentioned a few things they changed with the dampening and housing especially as it shares a lot with their Aviation HS-400. The comfort is good but this one has the most weight of all Beyers I encountered. Don't remember the T1 too well but it's quite noticable.
Edit: I have the PC360 as well and it doesn't hold up well, as in less transparency and less fun in the bass, yet still good as an allrounder.
Love my mmx's and you really hit it on the head with all of your points as to why it is such a great headset. My only gripe was the clamp force was a little to get used to, but that was cured once i got the gel ear pads. That's why I am skeptical to get the new Audio Technica headsets, because I am afraid they won't even compare to the Beyers....