Thanks a bunch, guys. I think I'll like these. Interesting to look at that graph again. I had the BW P7s for awhile and found a peculiar peak that seemed to be around 6kHz. It was occasionally piercing and unpleasant. Curiously, few others seemed to hear this, according to the various comments and reviews on-line. Either my ears are sensitive to that frequency, or I had a bum pair. None of my other HPs deliver that strange spike (not even the Beyer 990s). Consequently, that dip in the graph might just be the ticket for me.
I experienced pretty much the same thing with the P7. There was a lot to like about it, enjoyable low+mids and felt like very resolving high frequencies, but heard that peak you mentioned often enough to annoy me. Pity, actually tried it out 2 separate times, wanted to love it, very luxurious feel to it (including the packaging).
I found the NAD HP50 to sound bigger ("room feel") and more tonally accurate than the P7. If it wasn't for the comfort issues I experienced with the HP50 (earpads too shallow and headband that pinched top of my head), sound-wise I think I would be hard pressed to choose between the HP50 and 1540. (I wasn't able to try both at the same time, had the HP50 first and then the 1540.)
Still, happy I ended up with the 1540. They've actually become my preferred cans for watching streamed shows/movies straight out of iDevice.