Well, the short version is that the P7 has more bass extension than the 7520, especially in terms of quantity. Quality wise, I can't really say as my memory is not that perfect, but I'm leaning towards the 7520 being slightly more defined and more tight, it decays faster so it conflicts less with the next bass wave
The highs are crisper on the 7520, definitely, and the mids are more neutral on the 7520. The P7's mids are a bit recessed by comparison. Still, I'd describe the mids of the P7's as luscious, I've likened it to the way B&W's yellow kevlar drivers sound on their speakers (a fairly unique sound).
The soundstage on the P7 is bigger than the 7520, but I'd give the edge in terms of instrumental separation to the 7520. In terms of imaging and creating a sense of 3D space, probably the P7 wins there. I was amazed at how well I could place the various instruments all over the place, especially in classical music. I honestly haven't listened to much classical music yet on the 7520, so I can't fully compare that aspect here.
Otherwise, in terms of clarity, the 7520 wins hand down. As I mentioned in my 7520 review, all the headphones I've tried so far sounded veiled compared to the 7520 (and trust me, that's just because the 7520 sounds extremely clear, not because the other headphones are actually that veiled). Also in terms of detail I'd say the 7520 wins.
No matter how you put it, the P7 is a very good headphone. If it would've been perfectly comfortable for me, I would have kept it and stopped looking for another headphone. But now that I found the 7520, I'm glad I found that instead because to my ears it just sounds better
Yeah sure, all the detail and clarity has its drawbacks, as you will hear sooner when a recording is badly mastered than with the P7. But I can live with that