The Momentums are, what, $350? The B&O H6 is $400. So the P7 coming in at $400 is about what the market seems to dictate for higher end over ear cans.
That said, I've been in the market for decent over ears that are comfortable and have good isolation that sound good. Active NC or just passive isolation does not bother me as long as SQ is not heavily compromised. I have tried a bunch of them. My main pair has been the MDR-1RNC since last fall. I've audtioned the H6 - very comfortable, by my Sony Walkman does not drive it well so it is a non-starter because I'm not carrying an amp. I'm currently auditioning teh Harman Kardon NC which actually isn't bad, but the P7 intrigues me a lot. If I can get something that sounds good (to me) in the P7, is as comfortable as the H6, and is pretty portable, it'll be a great combination. Remains to be seen, of course.