Also found that the P7s scale very well indeed. It's actually hard to go back to going straight out of the iPhone/Ipod jack after using a good portable dac/amp (in my case the Fostex HP-P1) or a good desktop amp & dac (for me that's the Schiit Asgard II & Modi).
During the times when I have used the P7s with only the iPhone or iPod i've found that they can be fairly revealing of poor quality audio files, with AIFF ripped from CDs sounding good, while Spotify can sound pretty ropey on certain tracks.
Despite using the "Extreme" quality option for Spotify (320kbps) , it sounds like (and has been inferred by others) that many of the tracks are lesser quality that has been upscaled. The P7 is definitely capable of highlighting these differences, so my suggestion would be to, first and foremost, use the best possible quality audio files you can.
After that it's just a case of how far you want to take the P7s with extra gear, as KLJTech has rightly said.
All the best,
Edited by Jeb Listens - 1/7/15 at 1:19pm