For the p7 I have been using the fiio e12 diy, it's been a great combo through the line out of the x5, the p7 benefit greatly from a good amp, and one designed for clarity.
. But I think for the p7 the new fiio e12a has the ideal amount of power it has the perfect op amp and would be ideal for the job, giving the greatest amount of usable volume control
Yeah there's no doubt in my mind that the B&W P7's scale very well for a $400 headphone. They sound better (to me at least) when using the X3 along with the E12 and sound even better from the Gungnir and Asgard 2 combo. A friend of mine brought over his Bryston BHA-1 headphone amp and we used the Gungnir and Bryston BDA-1 as sources and the P7's sounded stellar with any of those combinations. There aren't a lot of closed headphones that I truly enjoy as I usually prefer planar headphones yet when driven by a good source and amp I think the P7's really shine. If I use my Sansa Clip Zip+ (love the Clip for the $) the sound is a quite a bit more closed in than if I use the FiiO X3 on its own. I need to get my hands on a FiiO X5, but thus far it hasn't been at the top of my "to do" list. With regards to home gear I find that the P7's sound GREAT with several of the Schiit Audio amps.