I won't have time to start the tread on headphone EQ which I intend until I finish a current project at work, but the essence of what needs to be done to do this correctly is this (I may get a few details wrong in summary):
One needs to match the response as heard through the headphones to that of a flat response speaker positioned directly in front of the listener. Doing so by matching loudness of each bark frequency band (think 1/3 octave EQ bands, but based on human hearing models) for both ears individually, keeping each frequency band centered pan-balance-wise as well as level-wise. You go through the process twice- first using the speaker as source, then again using the headphones. You then end up with two sets of EQ curves- one set of personal HRTFs for an out of head center-forward source, and another set personal response curves for the headphones you wish to calibrate as worn on your own ears. The personal headphone response set is inverted and the two are combined.
There is an app available which manages this and walks one through the process of doing the calibration corrections while listening to pink-noise and/or other stimuli. One runs through the calibration process and it creates left and right ear corrective EQ curves based on the both the headphones and the listeners personal HTRF response. You then listen through the app, or use the curves that are generated with other EQs.
The result is that the headphones are calibrated to sound just like a natural source located out in front of you. It is capable of generating out-of-head listening with accuracy sufficient for serious academic binaural comparison of the hall acoustics in actual spaces, meaning it's definitely good enough for us! Although intended primarily for accurate reproduction of binaural recordings, this should prove to be a real game changer for all headphone reproduction of live music recordings.
I have the app and information on how to do it, and have asked permission to discuss it here, but have not yet had time to do an implementation myself. Stay tuned.
And the doctoral candidate who did the software implementation of the calibration app for DG is a new member on this board, as well as a Phish fan.
Here's a presentation poster outlining the basics-