You may also want to check out http://www.head-fi.org/
While it is a more general headphone site, I've found that headphones that I enjoy for music also work quite well with my piano.
Just as a reference, I currently have: Grado SR80i, Shure SRH-840 and Sennheiser HD600.
Oh, and as to specs, none of the published specs really mean anything. More or less all headphones can produce all frequencies that humans can hear, but that doesn't tell you how well they do so, just that they make noise at those frequencies.
Ideally you'd put a few piano recordings on your phone or mp3 player and go listen to all the headphones you can get your hands on with them.
Oh, and because you asked, the absolute best is probably a Stax SR-009 with a Blue Hawaii SE amp.