Sorry to keep everyone waiting for my impressions but, unfortunately, I had to return my HP-P1 to Moon Audio due to a couple of problems I was having. I think the problems are unique to me and not the HP-P1 specifically, but in any event, it was not a good fit for me.
First, I discovered that my iPhone 4 apparently does not deliver a clean digital signal to the HP-P1 as I was hearing pops and crackles consistently while playing Apple Lossless files. As an alternative source, I have an iPod Nano 6G that I tested and it worked fine; hence my conclusion that my iPhone was to blame. I tried several things (resetting, airplane mode, different types of files (ALAC, AAC, MP3)) but could not make the problem go away. Again, this is a problem I could have overcome by using a different iDevice, but chose not to as I prefer to use my iPhone for the vast majority of my portable listening.
Second, my portable listening habits utilize iTunes smart playlists almost exclusively. I run my music library (currently about 8,000 songs) through a maze of smart playlists to achieve what I refer to as a "Daily Mix" of new releases and old favorites that I sync to my iPhone. Because I am mixing and matching new and old recordings, I rely heavily on Sound Check (I use the modified iVolume, which seems to work much better) to balance out the gain on my playlist. While auditioning the HP-P1, I came to realize that it disables Sound Check completely. Apparently, the iPhone (and the iPod Nano I tested) apply Sound Check after the digital-to-analog conversion, because it just doesn't work when you route the digital signal to the HP-P1.
These were deal-breakers for me, but like I said, they are really personal preferences and not indictments of the HP-P1 (unless the "crackling" issue becomes widespread).
As for my very brief time spent with the HP-P1, I was impressed with the build quality and sound. I only listened with my Edition 8's, so I can't give any opinion on harder-to-drive headphones, but the HP-P1 drove the Ed.8's with authority and accentuated the traits that the Ed.8's excel in: a very clean and highly articulated sound with tight and authoritative bass. Again, while I did not listen with any higher impedence headphones, I got the feeling that the HP-P1 is not power shy and should be able to adequately drive many full-size cans. Unfortunately, the HP-P1 was not a good fit for my portable listening habits, so I won't be able to opine any further. Others, I am sure, will find it fits their needs perfectly as the ultimate "all-in-one" audiophile portable DAC/amp. Their opinions of the performance of the HP-P1 will certainly be of more import to interested Head-Fier's.
Edited by NewMexiCat - 9/15/11 at 8:59am