I wanted to respond to some of the comments made here about the About.com Stereos measurements. I think too much has been made of the distortion I found in the Oppo PM-1 at 100 dBA.
I have measured 174 headphones at last count, and with about half of those, I also had some of Sound & Vision's writing staff do subjective testing of the headphones. Only in the most extreme cases did measured distortion correlate well with listener impressions. Usually that was with products such as cheap noise cancelling headphones, which sometimes do have audible distortion problems. I have only very rarely heard panelists complain of distortion problems with headphones at even fairly loud levels. I have measured numerous headphones with higher distortion on this test than I measured from the PM-1, and heard no complaints from listeners.
A test level of 100 dBA is very loud; I can listen to it, but I don't like to listen to it. And of course that level will vary at different frequencies due to the response of the headphone. So it might be as high as 110 dB or so with a particular sine wave frequency. (Even higher if a headphone has unusually large peaks and dips in the frequency response.) I chose this level because it's one that allows me to objectively differentiate the output capabilities of headphones. It's somewhat like CEA-2010 subwoofer tests. Almost no one pushes a subwoofer to 120 dB, but we have to push the subs to that level in order to measure differences in output capability.
I'm reluctant to judge any headphone by its measurements, or even to say what a "good" measurement is. There are a few I've measured that are clearly bad, with huge peaks and dips in FR, very high distortion, high sensitivity to amplifier output impedance, etc. But otherwise, I've so far seen at best an occasional correlation between listener perception and headphone measurements. And that's with frequency response. CEA-2010 perhaps excepted, our understanding of audio transducer distortion measurement and how it correlates to listener perceptions is at a relatively primitive stage. This is part of the reason why we don't see a lot of published audio transducer distortion measurements.
Based on what I've heard of the PM-1 (soon to be published on About.com Stereos -- shameless plug), there are good reasons why you might choose the PM-1 over a competitor or a competitor over the PM-1, but distortion isn't one of them.