How Do Amps Affect Soundstage?

I have heard very good sound staging with otherwise bad sound. Distortion will not always reduce the size, width, and depth of the sound stage, although it usually doesn't help. It might sometimes, for some people. I've had the soundstage pop into place with a little equalization, proper application of a house curve. For the sound stage of a 2 channel, 2 speaker system, there's abundant  interaural crosstalk that is unnatural and difficult for the ear/brain to interpret consistently. How that gets interpreted depends on a lot of factors and can make it tricky to get a good soundstage, which may happen for one person and not another listening to the same system in the same sweet spot.  Addressing interaural crosstalk means doing something beyond the orthodox 2 speaker listening triangle. My experience is that there's a hard upper limit to what can be done with just two speakers in terms of sound stage. You may find something that works really well for you. Some things that seem to always help are well matched speakers and amp channels - as close to identical as possible, perfectly level matched. Good off axis performance of the speaker can also help, and dealing with early reflections by carefully placing the speakers and judicious application of room acoustics. I think it generally helps to have a decent distance from the speaker to adjacent wall surface, which will provide some delay and attenuation of early reflections. The 2 speaker listening triangle is a delicate and fussy thing that presents the ear/brain with a signal that has inconsistencies in terms of soundstage and tone that can at best be mitigated.