I think now it boils down to personal tastes. I listen to West Coast and Cool Jazz, Bossanova, classical (mainly strings though), up to current R&B, hip-hop and modern vocal jazz. A friend of mine has the P5 of which I listened to for an entire afternoon. Personally I found the P5 too dark for my tastes. My Ed8's are the extreme end where it's quite bright.
The P5 has a great bass (esp amped!!) but the treble lacked the detailed sparkles I'd like to hear in especially my jazz.
If you get the chance, and if you're looking for headphones for the HP-P1, then say take a look at the DT1350, HD25's, etc for the price range you're looking at. the M-80 wouldn't give make your HP-P1 sparkle 'cos it's good enough un-amped. If your budget goes up higher, then at least from my tests, the Ed8, high end Custom IEM (UERM, UM series), etc. would also bring out more in your HP-P1.