Seeking Isolating Low-Impedance Circumaural Headphones

I currently have the Sennheiser HD650 headphones and I love them. However, I would like to find a pair of headphones for on-the-go use that are circumaural (more comfortable to me), low impedance (<100, suitable for mp3 players and doesn't require a pre-amp), and isolating (so I can listen to music on the plane without much outside interference). I listen to a wide variety of music including techno, classical, new age, hard/soft rock, and alternative. I really like bass, so I'd prefer to have a pair of headphones that doesn't sacrifice bass (they shouldn't overpower bass either).

In order of importance:
Sound quality

Comfort

Isolation

Build Quality

I read the shoot-out thread but the most promising one on there, the Sennheiser HD25-1 II, isn't circumaural.

Thanks for any suggestions!

I decided to go for the Audio-Technica ATH-M50 after reading that thread some more. While they do isolate a bit less, their comfort is rated higher and I'd much rather have a pair of headphones that's not clamped on top of my ears during long listening sessions.