Note that while there is a very close relationship between frequency response magnitude and phase, if you go back a few posts(http://www.head-fi.org/t/612665/how-far-can-eq-really-go-towards-truly-equalizing-headphones/75#post_8455577), it is evident that looking at the whole picture (phase and magnitude as opposed to just magnitude) is important to determine how the output signal is going to behave!
Now: How far can eq really go toward truly equalizing headphones?
If the headphone is well behaved: relatively time invariant (positional invariant here might be a challenge though), very linear, and minimum phase... Well pretty darn far. Make a filter with poles and zeros where the headphone's transfer function (z-transform) has zeros and poles respectively and you might get a flat line (where the headphone can reproduce signal of course - best not to equalize close to perfect notches there for SNR considerations). Furthermore, cascade a second filter to tailor things to your liking... Not saying all this is super easy though. Note there are positional variations in response, THD, SNR issues... Just, I can see EQ can help to a good degree to the best of my knowledge.
Edited by ultrabike - 6/14/12 at 6:09pm