Human hearing is generally more sensitive to upper midrange sounds, so excessive amplification/boost in these frequency bands can be too much for many listeners. Thats why car and building alarms, bomb-raid sirens and car horns contain a lot of acoustic energy in these bands. Fingernails or a fork on a chalkboard...IIRC is pure upper-midrange acoustic energy.
If you look at the sound signatures on the headroom build a graph site most of the head-fi favorites, or those noted for having an overall "pleasing" sound have some degree of upper midrange recession from ~2~6Khz, accompanied with a slight bass/treble boost. Just look at the difference between the PS500 and RS1... and the general head-fi commentary surround those two.
If you wiki human hearing and auditory nervous system theres a TON of info on the subject. But thats why we see what we see from the headphone manufacturers.
Edited by kramer5150 - 1/23/13 at 12:03pm