why do expensive amplifiers produce a better soundstage

Can we state that a greater expense correlates to an improved signal to noise ratio? Or expense might be part of pricier bits inside the machined metal box?

I understand that there are exceptions.

@yamaho wrote:

why do expensive amplifiers produce a better soundstage

I don't know why this is naturally assumed; they don't necessarily. On the other hand let a given amp run its speaker load actively without the interference of a passive crossover between it and the speakers (which naturally requires more amp channels for each driver section), and then you would have a more resolved, cleaner and more effortless sound altogether, which could aid the acuity of the spatial aspect. 

@russbutton wrote:

It is a physiological response to wallet depletion.

Good one. 

OP why do expensive amplifiers produce a better soundstage

@phusis I don't know why this is naturally assumed

+ 1 it’s not automatic. Speakers also need to be up to the task, placed optimally, and likely room treatments to better pickup soundstage cues. Maybe also good channel separation in the audio chain.

Although higher parts/engineering/price usually yields better sonic results, it’s more the execution rather than price that contributes to better soundstages.  

Soundstage is an acoustic/psycho-acoustics concept...An amp can contribute but cannot create it without working with the room acoustic parameters and without working with the psycho-acoustics parameters..

No more than a room and your brain/ears can increase the details of a digital format but only contribute to his resolution  and perception...

The really expensive ones” because they address both electrical ground and mechanical ground issues. . 
Goldmund, Gryphon, CH Precision, dartZeel off the top of my head.