OK. Did a bake-off tonight. All Sennheiser phones: HD555, HD530-II, PX-100, HD25-1-II.
I would give a bit more credit to these phones. You slam them on, and you go, "whoa, why don't my other phones cover the bass like that?!?" So yeah, nice linearity. On bass and drum tracks, you pick up those instruments and the playing really well. In general, they seem pretty linear and well balanced.
In terms of sound isolation, they are hard to beat. I really bought these for the office and for airplane travel. And none of my other phones isolate sound as well as these do.
As for my own preference, I'm tending to like the phones that give a bit more air at the top end, and let the mid-range sing a bit more. The HD25-1-II doesn't cover that as well as my HD530-II, at least for my ears. And my ears definitely ain't the best around.) My ultimate phone probably has more of the bass extension and bass definition like the HD25-1-II, but with some of the sheen of the HD530-II. (T1 maybe?)
Pulling the black pads seems like a good suggestion. Will give that a shot as well.