I've found another factor when EQ'ing headphones that screws everything up-ear canal and pinnae size.
I built a binaural microphone. Recorded over 40 headphones and analyzed frequency responses:
My first design had much smaller pinnae and ear canals:
But when I switched to my present design, with larger pinnae and deeper ear canals, not only did the frequency responses for all 40 plus headphones change, but there were also soundstage...