So the notion of soundstage being a property of headphones certainly has a lot of substance behind it, and indeed, even untrained ears can anecdotally easily perceive this. However, the theory behind is rather confusing to me. Since soundstage seems to be related to the openness of headphones, it would imply that the interaction of the outside world and the user's ear is at least partially responsible for the improved soundstage. Now, this raises a question that I have been struggling to find conclusive answers to: If soundstage is a result of audio waves being "colored" before entering the ear, wouldn't this negatively affect audio that has already been prepared for headphone experiences? As sound being recorded with a dummy head goes through the filtering that our own ears perform, surely any playback not bypassing the human exterior is going to suffer from excessive coloration, since one would unnecessarily combine the HRTF of the recording with the more minimal HRTF/PRTF associated with headphone playback. Conversely, with "pure" recordings that haven't undergone any HRTF, e.i. virtually all music, audio will benefit from a layer of extra spaciousness introduced by end-user equipment.