I heard these for the first time last night while shopping for a new large screen TV at Best Buy. They were next to one of their two higher end sound rooms (which I didn't even know they had), and I put them on while waiting for my purchase to be rung up and was really blown away by the sound with the amp on but the noise cancellation turned off. I really like that you can listen to them in passive mode, with the built in amp on but w/o noise cancellation or with the amp and noise cancellation on.
I have no idea if the headphones are truly THAT good or if its simply due to them playing a certain selection of music that really shines with those headphones or any other headphone they sold on that wall...they were next to the B&W P5's. The PSB M4U2 were attached to the unit that only allows you to select play and volume up or down but they certainly did sound incredible. I don't own any closed back can's and have no need for noise cancellation BUT these having me seriously considering adding them to my headphone collection.
I'm really curious if the PSB are truly great for the money or are simply nice but sounded great with the music they were set up top play. It may be that I was SO tired of dealing with sales people that any good sounding headphone would have been a wonderful break at that time...I have a feeling that the PSB is something special though. I'd love to see a review of them compared to other cans in their price range like the HE-400's, AKG's...something along those lines and based only on SQ as I couldn't care less about headphone collection.
They are truly an excellent headphone. You may also want to check out their "sister" headphone from NAD, the HP50. They are $299 with better looks, comfort and similar sound, just no amplification or active noise cancelling.