I have also commented about the role that individual anatomy and the "interface" between human and headphone probably plays an important role, but I am not so sure this applies to the degree you may assume. It does seem logical, but are there any studies that confirm we might here close proximity sources so differently? I would be very interested in learning more about that. I don't think different ear shapes process sound waves differently enough to account for a change in perception between average people. Yes there is a proximity effect in the world of microphones, and perhaps our ears are subject to similar reactions, but my assumption has been that bass response is clearly affected by the seal and/or the volume of air being stimulated along with the distance from driver to ear... and that different shaped heads and ears would affect this and account for why some people have very different takes on bass performance... AKG K550 being a great example.
I still think we are all mostly hearing the same things regarding mid and treble frequencies, but our source material and electronics chain are both important variables as to how the objective response of a headphone driver translates to subjective perceptions by the listener. I would argue that source is most important, then followed by circuits (DACs and amps) then followed by "wires/connectors". I think it is interesting to consider that "anatomy" may in fact be a significant variable, and more interesting still to consider how to qualify and quantify. There are clearly some folks looking at this very closely, like Tyll or Purrin, and my own take is that psychoacoustics and maybe a general proximity effect common to the vast majority of human ears accounts for emphasis often found below 1 kHz, the dips above 1 kHz, and then some emphasis at some point (or multiple points) above 4 kHz. I also wonder how much of the deviations from measured neutrality or purposeful by the design/engineer team and how much is just the result of wrestling with a driver that is being stressed to do it all. In the world of single driver loudspeakers, this is clearly a constant challenge borders on alchemy to get it "right", with the definition of "right" being different for a variety of audiophiles who will obviously not share the exact same values/priorities in music reproduction and listening.