I own the HP50. Chose it in direct comparison against the original Momentum, PSB M4U2, and the B&W P7 in an extensive audition, just ask my fourteen year old daughter who had to hang out in the store with me while I performed the comparison. The comparison consisted of those specific models as they were the closed back portables in the smae price range that the store carried other than the HP50's. Took my HP50's to TAVES last fall and spent more than an hour comparing my HP50's against three different prototypes of the PM-3 using my FiiO X5 and there was no comparison. Nowhere near an hour was required to determine that the performance of all three of the PM-3 prototypes were on a different level than the HP50's, and I love my HP50's. I spent some time with the HA-2, not all that impressed, and the HA-1, want one, as well.
I truly don't think it would be out of line to say that the Oppo PM-3 is probably one of the five best closed portable headphones in the $400 US range and below on the planet currently. No I haven't heard nearly all of the candidates, but who has? It's an assumption, but judging from what I heard that Sunday back in November I think it's a pretty safe assumption. Of course there will be those who disagree and they may even come on here to tell me so. That's fine, but i would suggest that such a conclusion would simply be the result of personal preference more than anything else as is my own.