To me sound-stage or more accurately soundspace imho (in a headphone, not the recording) is the X/Y/Z space in which you hear the music. If you envision that space like an onion, the layers are the layering. Imaging is the movement of sounds within this space, imaging can be fuzzy, or precise, like a hazy left-right sound, or a marble floating through the air.
To me this soundspace is completely seperate from the recording / stereo image, since you can still hear it in one IEM or headphone cup at a time, as evidence.
The Audio Technica CK10 introduced me to imaging, at which point headphones started to sound lackluster. The Shure SE425 is a good example of imaging too, I haven't heard speakers sound that precise, but the sound-field is much more enveloping of course, and you need to take room acoustics into account.
A speaker junkie will use cross-feed to emulate speaker sound in headphone's or IEM's.
Afaik there is no technology or software which can measure the soundspace, layering and imaging characteristics within an IEM or headphone, those Brüel & Kjær dummy heads only measure frequency response (volume balance) and square-wave response is not sufficient, clearly evidenced by data like this (two extremely different sounding IEM's)
To me, this is one of the reasons I find audio fascinating, it's an inexact science, still unsophisticated in relation to what we hear, and there is no speaker system yet which is 100% transparent, i.e. they are all coloured in a sense.
There are some scientists which like to purport that audio is a complete and finalized field, but then I don't see why there is such an extreme amount of controversy, compared to a field like video cameras, or mathematics. Audio is more like medicine to me.
Edited by kiteki - 4/29/12 at 7:54pm