Im afraid im not following your point....
An amp should play the source material back as recorded. It should not change anything about it.
If one amp is different from another in power or voicing, such as equalization, that is more normal.
If an amp widens, or narrows the soundstage for that matter, there could be something out of spec about it.
For instance, my preamp has a width/blend knob, which can sum or difference the channels. If its out of adjustment, the channels can crosstalk to one another.
Alternatively, if a transistor or out of spec capacitor or some other thing is changing the sound either in the preamp section or the amp section, it can make for some strange artifacts.
I once had a digital synthesizer that seemed to sound like it was three dimensional compared to others like it. It had other issues, so I reseated the chips and re-capped it as well. That "effect" went away when I powered it back up.
I also had a Scott receiver that the mono button was acting up on. It would occaisionally reduce the stereo effect unless you messed with it for a minute.