I take a small amount of umbrage specifically with the term "headstage" because it's a play on the word, "soundstage", which, to me, has a very specific meaning of the abilities of speakers to recreate the space of the original recording. Soundstage is, of course, an illusion, the aural equivalent of stereoscopy. Short of binaural recordings -- which no-one is ever talking about when they're talking about headstage -- this is even more illusory with headphones. It is my humble opinion that most people who hear "headstage" are deluding themselves even more so than usual.*
It's interesting that you talk about placement -- I don't think there's anything wrong with being able to place sound elements anywhere "within" the head -- anywhere between the two driver elements -- that can be done simply with balance. "Headstage" would require either binaural recording or some sort of HRTF (which, again, people usually aren't talking about -- this entire discussion changes if that is indeed part of the discussion).
But that's "placement" -- the ability to place sound elements in the sound field.
Also, that's not "separation" -- the ability to separate sound elements in the sound field, especially those which are close to each other.
But both of these can be accomplished with normal, untreated recordings, whereas to get a soundstage via headphones, one must be listening to a binaural recording, or a regular recording run through a HRTF, or some such.
*Self-delusion is something all of us do all the time to a lesser extent, so I'm not trying to imply that deluding oneself is, in and of itself, bad. The ability to hear a soundstage requires a certain amount of self-delusion, for example -- one must be able to close one's mind's eye to the fact that you aren't actually in the original recording venue.