Just got my pair of UE6000's in. They have a really great box, and the 3.5 jack splitter keychain dongle that they have is really nice. Great accessory set, Logitech. I disliked the cable though, so I'm using them with a NuForce Transient Cable.
Initial observations (in passive mode): these things are p-o-l-i-t-e compared to the Amperiors and the M-100's. They actually remind me a lot of the Momentum's that I used to have around. At the $90.00 I paid for them, they are so much like the Momentum's in sonic character that I'm really now saddened by the Senn's price point. Lower mids seem fine, but upper mids feel really recessed. Nice detail, nice very bass (the bass gets boomy with the active mode on, but I find it a reasonably pleasant kind of boomy.) Listening to Beach Boys Pet Sounds "Sloop John B.", the high range is there -- but not sibilant or sharp at all.
Their polite character really is unpleasant for listening to Crowded House, Split Enz (really, anything by Neil Finn) or Talking Heads. "Into the Sunset" off One Nil seems way too relaxed, especially on the chord change to the outro. The Cars "Just What I Needed" lacks punch - it feels slow. The woodblocks that open Talking Heads "Listening Wind" seem incredibly far away. I'd chalk that sense of increased distance up to the difference in driver distance to the ear...except that it's just not the case with the M-100's with XL pads which are almost equidistant. (That's also not the case with my HD580 & D1 combo, and those things are earmuffs.)
Going to use these around Chicago in passive and for noise canceling for a few days to decide if I like them or not. If not, they're getting returned. I get the sense that they may be a bit too laid back for my tastes, like the Momentums, and won't be hanging around too long. When you said that you "loved the clean forward mids of the amps" that's totally me too. Where the Amperior was too uncomfortable for you, I don't have that problem -- I'm just looking for an over-ear that's similar in character for long, at home listening sessions. The hunt continues!