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?

Surely, those expensive produce a better soundstage. However, this is not. 

The DIYer's do have passion in building the super amp with top notch performance.