I like how Joe B sees things. there are some limitations to this, but trying to get rid of some electrical or acoustical dampings and replace them by EQ might actually work for the best in lots of situations(desired reverb excluded). but it's a real effort to get it right and it would probably make most give up.
audiophile hatred of EQ is a mix of ignorance and intellectual ideas of "real sound". acoustic chamber, phase mismatch, electrical damping, pressure dampers, choice of this or that driver, choice of an amp or a source only based on how it sounds, tube rolling, cable "upgrade"... it all looks perfectly hifi to people for some puzzling reason, but turn a knob yourself and you're galactus the devourer of sounds.
and of course people taking us for fools have actually no idea how much distortion is involved with a good EQ and how ludicrously low it can be compared to almost everything else.
@flisker. the point is to admit that EQ can improve you enjoyment of music and correct some defaults in a given headphone, instead of having to try hundreds of phones in the hope of one day finding the perfect candidate (and by that time you would probably have changed your source anyway).
I agree with you that it would be challenging at best to make one headphone sound exactly like another just with EQ. but that's not what we try to do. if I preferred the hd700 to my relaxing hd650, well I would buy a hd700.
also just trying to reverse engineer the sound of one can into another using both FR graphs is bound to fail. FR graphs don't take a lot of the sound into account, distortions of all kind, and the way one given frequency will rebound in the headphone and affect the quantity resulting, by cancelling itself or multiplying. none of those are on the FR graph. and it will make some headphones great to EQ, and others almost impossible to tweak.
only by listening can you get really close to what you're looking for(well I'm sure a good measurement system and some acoustic software with the 3D schematics of the headphones could do it better, but I don't happen to have any of those ^_^).
for bass tightness, you can't make the coil start and stop moving faster that is does (given that the source already provides what it should at all time). but you can play around with sub bass and bass, and fool your brain a good deal. but then reverb can't be simulated with EQ, and some cans just can't make some frequencies without unbearable distortions. so EQ is not an all in one solution, but it can do a lot more good than it does bad.
Great answer, In this case I agree of course.