The frequency response specs for headphones don't actually tell anything about how a headphone may sound, just the frequency extremes that the phone can supposedly reach. They're also not very useful for comparing phones because manufacturers test under different conditions and with different tolerences.
If you want to hear how low a phone can reach before you buy it, I suggest that you compile some test tones or tracks with low frequencies on a CD and listen to it through the phones. Trust your ears, not the manufacturers' specs.
Many a $5 pack of headphones claims 20Hz to 20kHz response. I'm not really sure what qualifies as a "response" in this case...
...and that's exactly the problem. There is no generally followed standard for what counts as a response for the purposes of that spec. It's completely arbitrary.
It's a pretty safe bet, though, that even if headphones can produce a 15Hz sound, it'll be a lot quieter than they could manage at 200Hz, even ignoring whether a human listener would hear the 15Hz at all.
) they forget to say at what levels the extremes come through... yes they move, but so little you won't hear anything. given values are only a little usefull when they give the deviations (eq +- 3db means something like the given extremes are at most 3db lower than average).
they'd probably get away with stating 0-100000hz (at +-100db levels...)
I don't know really, I think people can hear about 40db difference in dynamics if they try hard, but it does mean it'll be considerably lower than the rest of the music so you won't hear much of it. compare it to HD600:
one kind of standard i think (with speakers mostly) is measuring the +-3db points. still does not say much about the sound though.