I own a pair of M50's, as well as a pair of B&W P5's. (And a lot more IEM's and Ciem's) Today I went to listen to the P7's and I also listened to the M4U 2's. I was surprised at how unimpressed I was with the P7's as I love my P5's and my C5's too. I didn't care for the fit or the sound of the P7's. I found it lackluster at best. I quite enjoyed the PSB's though. Very comfy, very nice sound with good isolation. Then I tried some Sennheiser 700 something or other and didn't care for them. The Sennheiser 650 was my favorite of the day, narrowly edging out the PSB's. I don't listen to my M50's much, they are uninspiring. I was surprised to read a comparison here that strongly favored the P7's. I may have to try another pair.
My source was a Fiio X3 playing all FLAC music. I was worried about the sound until I tried the PSB's and the 650's. Yikes.
(I bought nothing today, Hooray! Hooray!)