Mapman and Wolf, I am only bouncing ideas.
My point is that if speakers placement has been optimized properly, one should still be able to detect small differences in soundstage when changing amp because of the reasons I have stated above. Of course, we talk about small differences. Further, because each amp controls the subwoofer of speaker differently, the speaker position might also need to be slightly changed when changing amps. Therefore, because most of these small differences are cause by the interactions between amp, speakers and room, IMO it is basically impossible to determine them by performing measurements on a single component (except maybe for point 3). One has to try the various amps in his room.
Wolf, some example: left channel has slightly higher distortions/volume then the right channel. I clearly stated that I do not know I significant/large such differences can be. Thus, it should have been clear that I was only a guess.