Maybe it's on a far horizon, to either quantify or categorize headphone characteristics that systematically affect imaging and other spatial cues. I'm kinda new to this, but I recently spent more hours than I'd like to admit doing level-matched, bi-directional (although sighted) A/B comparisons between headphones, partly on spatial characteristics, and it's made me curious to learn more. But I despair a bit about sorting out the variables that must include the various spatial cues in the original recording, the headphone geometry, my HRTF and the psycho-acoustic processing in my own brain, which itself probably includes not only direct perceptual processing but higher-level expectations about the music, the recording and the equipment.
That said, when I change just one variable, the headphones, the spatial characteristics of the listening experience change. Something specific to the equipment does attach to spatial impressions.
Maybe I'm over-complexifying it. But maybe I haven't educated myself enough. You're pretty sorted out on a lot of these topics, Robo9999 ... I'll look, but if you know a good reference work or two, let me know!