why do expensive amplifiers produce a better soundstage

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. 

Adequate power to negate frequency shifts due to speaker impedance variations.  This doesn’t necessarily require that the amplifier be expensive but some things to look for are a robust and quiet power supply, adequate power for the sensitivity of the speakers being driven, and the ability to nearly double power into lower impedances, which is more critical when used with hard to drive speakers.