why do expensive amplifiers produce a better soundstage

I agree with amplifier design first. Then power supply and component quality.

@emergingsoul 

Thank you. I am indeed fortunate.

@lanx0003 

"...streamers and DACs—effectively collapsed depth and masked layering, resulting in poor separation and diminished spatial cues."

"At least in my system, and based on my listening, the primary bottleneck was clearly in the digital front end, not the amplifier."

This was our biggest complaint with digital early on, it was the DACs not the recording or the format.

@fiesta75 

"I agree with amplifier design first. Then power supply and component quality."

What's coming out of the speakers is the power supply being modulated by the source (music).

Musicality and soundstage.

I think it’s a question of balance, gleaming the best from all of your systems component parts. Not to be forgotten is the room, it is also becomes a musical instrument. A rooms musical/acoustic footprint finally defines the soundstage.

As an example, music played with speakers placed 10 yards apart will still produce a musical black hole, even with $10,000 interconnects.

"What's coming out of the speakers is the power supply being modulated by the source (music)"

on my variables importance pareto chart, speakers+room-acoustics are higher than amp. PA system, including amp+speakers+room, has to be characterized combined, where time and frequency domain audio transfer function is defined between amp input and sound at listener ears points.