I ended up settling on the UE6000, mostly based on the Inner Fidelity review and based on my time with them in Best Buy. I am using them in active mode in the office, with my Leckerton portable amp, and also as an offset to the airy, bass-lite DT660.
I would call them balanced and neutral, with an eye towards being slightly dark. Mids sound slightly forward and have good imaging (vocals easy to make out, underlying guitars easy to follow). It responds well to heavy metal guitars and their groove, something I was really looking for and that the DT660 was incapable of. Bass sounds good and tight with the amp, very slight boom. Important part is that it doesn't bleed into the mids. My Leckerton amp tightens up the bass nicely. Treble is tastefully done, not much sibilance, and it has a better attendance record than on the Momentums. I think their imaging is pretty good, I can picture where the sounds are coming from on the soundstage pretty easily.
They are extremely comfortable, probably the most comfortable headphones I have (HD800, AD2000x, AHD2000, DT660, SR-80). They feel like I have nothing on! I can wear these all day easily and my ears easily fit inside the earpads (an issue I had with the Momentum and K550). I have no plans on bothering with the noise canceling, the isolation (both ways) is perfect for an office. Not sure about a commute or anything. Not having the batteries in also makes the headphones lighter.
The only thing I dislike about these guys is the cable. It's short and looks easily ripped. I would replace it in a heartbeat if an upgrade were available, just to have some extra length.
For the price (almost $200 less than the Momentums!), for the versatility and for the overall sound, UE6000 is a real (and surprising) winner.
