Agreed on the comparison to the HE-6, and alot of the better desktop rigs in general. It's not really fair to put the P7 against such a setup, but having a basis for comparison is nice. My desktop rigs consists of a Peachtree DAC-ITx feeding a Schiit Lyr with upgraded tubes feeding the HE-500. So while it's not fair to put a $400 sealed portable headphone being driven from an iPod up against a $1700 rig, it is nice to have a comparison.
When I compare either the Momentum or P7 with my desktop rig, I find the latter to be better at everything. THe soundstage is wider, imaging is better, bass is more controlled and yet still plenty of it, and the mids and highs lead to instruments sounding more realistic. The desktop rig is better in every way that matters to me.
What's surprising, however, is that the margin of victory is not nearly as large as I'd expect. The desktop rig certainly doesn't sound 4 times better, and it doesn't even sound twice as good. If I were to have a scale of 0 to 100 for sound quality with 0 being basically no sound and 100 being the best sound I could imagine or the perfect sound for me, my desktop rig would probably fall somewhere around a 95 and the two portables I have in hand now around the 90 to 92 range. While the absolute values of those numbers are somewhat arbitrary and maybe not particularly useful, the relative distance between them may help give an idea of where I think they fall.