There are no graphs that show imaging of a headphone. First off headphones never truly image in the first place. There are factors like frequency response, clean decay, driver matching etc that help the headphone convey a really crisp stereo image. Anything else is a component of psychoacoustics, and it just so happens that the open-ness and frequency response of the HE-400 allows to have a sound that can extend out rather deeply and layered at times.
I'm sure distancing the driver a bit has something to do with these companies' choices for their headphones, but I'm also pretty positive that fit has a lot to do with it as well. Oval shaped angled pads that are thicker in the back just fit better than round, flat pads like on the HE series. Our ears are naturally oval shape and thicker in the back. The HD800 is really the only headphone I know of that has a very expansive sound that also doesn't give up on definition and clarity. I've not heard any of the Stax though.
Yes I tried fully modding a pair of pleather pads I didn't use anymore into jergs.