actually a freq. response of 5Hz to around 40kHz is better but you'd have to pay a LOT of money for headphones with that range of freq. response.
besides, humans can't hear much of anything below 20Hz or anything above 20kHz.
however, many labs have said that most people's hearing range is anywhere from 16Hz to 20Hz, so unless you have superb hearing there isn't much reason to go much higher or lower.
[Source]1kHz = 1000 Hz btw
Edit:
Though people can feel, in the body, as low as 4Hz, which is kind of a cool sensation.