As someone who prefers accuracy in reproduction I have always wished for a more objective approach for achieving it with headphones. Unlike with a loudspeaker/room system, the sound produced in the headphone/ear interface is not only dependent on one's individual auditory canals, it doesn't lend itself to easy measurement and correction.
So I was quite intrigued when, some time back, I ran across Siegfried Linkwitz's webpage on "Reference earphones," where he found that the perceived frequency response of headphones (relative loudness changes determined by sweeping over the frequency range with an audio signal generator) typically exhibited significant - but narrow - peaks, which could vary from person to person.
Linkwitz's suggested solution was a simple passive LCR notch filter circuit - to be placed between a headphone amplifier output and the headphones - designed to achieve a uniform perceived amplitude response for a particular listener and pair of headphones/earphones.
See the complete post about custom EQing headphones on EDN.