Righto....FWIW, I hear zero noise floor on either my portable or desktop amp.
I wouldn't try it. My Lyr took out my Ed.8s (another member had the exact same combination). I would only recommend the Lyr for orthos and definitely not for low impedance / highly efficient dynamic headphones like the P7.
Were you listening to the Ultrasone's Edition 8's when they were ruined by the Lyr or did this happen simply by plugging them in?
I've had zero issues with any headphones I've used with the Lyr including easy to drive phones like the Grado 60i, 325i, RS1 and even a couple pair of IEM's (Westone 4R, Klipsch X10 and X5's). Obviously the Lyr is a very powerful amp so I'm always careful with how quickly I turn up the volume, and I have had issues with some low level noise with IEM's (which the amp isn't meant for) but other than that no problems whatsoever. I prefer it to the all solid state amps that I have on hand for most of my headphones.
I read somewhere here that there may have been changes made to the Lyr since the earlier versions that prevent the problem you had, but I have no idea if there is any truth to this or not. I do appreciate the heads up, it may be very possible that the noise floor will be much higher with the B&W P7's if driven by the Lyr...I have no idea as I've only heard them in the store, and that's hardly the best way to judge any pair of headphones. In the store I was very impressed with both the P7's and the PSB M4U-2's. One would think that the fiber driver/speaker (with a surround) in the B&W's would require more juice than your typical mylar style driver found in most non-planar style headphones, but maybe not.
I'm torn between the P7's and the PSB M4U-2's...I much prefer the looks of the P7's over the PSB's based on store demo only...for whatever that's worth.