Hi Jonathan,
I planned on building a headphone system for work but it never materialized, and I have a pair of HD600's in like new condition that I can part with if you are interested. I tried quite a few different phones, AKG701's, Shure's, Ety's, and AT's (not the set mentioned above), before settling on the Senn's - for me they sounded the most natural and airy, and were the most comfortable to wear by far.
I've seen frequency response charts of the HD600's and they were flat to 30hz and gradually down to about -5db at 20hz - supposedly even better in ear. They are rated to mid-teens, but they do not over-emphasize bass response, which some others do - sounds good at first, but gets tiring pretty quickly for me. As one reviewer put it,
" The exceptionally natural quality of the HD600's overall sound bears reiteration, since so many headphones with "fat" or even "slamming" bass response deliver the low-end goods with a degree of emphasis that, while pleasing to bass aficionados, grossly distorts the frequency balance, limiting the range of music and recordings that will sound good on those headphones.
How does Sennheiser do it? Through computer-optimized magnets--critical to driver operation--that minimize harmonic and intermodulation distortion, granting the HD600 its uncanny clarity and spaciousness. Furthermore, the headphone's advanced diaphragm design eliminates standing waves in the diaphragm material, preventing unwanted emphasis at otherwise resonant frequencies."
The AT's mentioned above sound like they are good as well, but I've never heard them and can't compare for you. Shoot me a PM if you'd like these at $175 plus ship. (I've the same member name at Audiogon.)
Chris
