why do expensive amplifiers produce a better soundstage

It has very little to do with price and a lot more to do with designer's niche knowledge. For example, I know a less than 2k amp that is conducive to a deep expansive soundstage and a 50k amp that fails at it (designer iq low, ears bad)

Choice of components, design of the circuit itself, signal pathway, etc

On the same note, amps can be made to measure the same, but, sound quite different, just SS designs. The low IQ measurement crew will struggle with this one...

why do expensive amplifiers produce a better soundstage

The above mentioned is where amp design can be considered an art form, not just grunt engineering.

Sub-par transformers, circuit layout, and attenuators/volume controls are where soundstage gets lost in many preamps. The conundrum is that some tubed pre-amps add artificial soundstage through second order harmonic distortion. The same second order harmonic is perceived by many as depth and holography. A little judicious added second order harmonic is a good thing-the ARC Ref 6 and 10 are examples. A bit too much creates more problems than usual with gear matching-the Primaluna Prologue Premium is an example.

I was led to believe that, expensive amp's had luxuries like silver solder. Rather than tin-lead s**t. Why has no-one mentioned that?

@irjones 

Because it probably has negligible impact on imaging relative to other influences?