why do expensive amplifiers produce a better soundstage

Rolled off high frequencies alone will typically widen the soundstage, so be careful about that. 

@toddalin 

What are Merman speakers? I never heard of them.

+1 ntpc4

It has been my experience that a great tube preamp does help foster soundstage. Since the preamp must mate well with the amp they both can play a role. 

Price means nothing. Low noise floor & crosstalk between channels. 

Emphasis or lack of emphasis on one frequency or another can alter the perception of the "stage"

My subjective opinion on my personal experiences is that tubes alter this perception to a higher degree than the S.S. components I have used. Tube rectification vs diodes when trying both in the same type preamp circuit has an affect/effect? as well. I favor tube rectification in preamps. 

Ultimately the recording & speakers/room set up hold the biggest key to stage perceptions. All of my comments are solely based on my opinions