why do expensive amplifiers produce a better soundstage

What the heck does soundstage mean? I don't think many people really understand it.

... life is pain (brain fog, dilate eyes, more body aches, never satisfied with music, requires lot more money, etc.).

You’ve described my condition with remarkable accuracy. 

everybody knows that $10,000 interconnects produce the soundstage.

If we are going to include the room in the mix, we should also include the type of speaker since line source speakers are less influenced by the room than point source speakers.

+1 @lanx0003 Spot on.