why do expensive amplifiers produce a better soundstage

Expensive Power Amplifiers, in both Valve and SS, with the SS Amp' costing approx' £45K and the Valve Amp' costing approx' £20K as Commercial produced models. These are models experienced by myself in a familiar system, with £75K Speakers in use.

This experience had, does not support the notion stated in the title.

" why do expensive amplifiers produce a better soundstage " 

In the same system I have heard a readily available SS Power Amp' that cost much much less, but deliver as a performance everything the expensive models were capable of. If an extended listening period was available for the comparisons, maybe the more affordable model, will have proved to be the better of the Amp's during the comparisons?

In other systems I am familiar with, I have heard the affordable Amp' compared to other Commercial produced Valve Power Amp's that come with a cost close to £12K, with the same results already having been experienced.

In todays world of Audio Equipment, a person does not need to extend to uber monies, to experience a extremely high quality audio presentation.

I am at the current time, with a affordable model Amp' to be tried in my system along with my owned Valve Power Amp's.

Another Amp' come under my radar and comfortably seated in the affordable arena is the Models in the Link.

https://forum.audiogon.com/discussions/clone-dartzeel-amps

Limited to reports on this forum only.  This Amp' for certain users having taken a punt on the purchase. Has proven to be the Amp' that has superseded the use of Amp's up to $15K. My experience suggests, an Amp' that comfortably supersedes commercial designs up to $15K, has the capability to stand toe to toe in comparisons with much more expensive Amp' models from the commercial world. 

During 2026, I will be extending my investigation of the Amp' in the Link, and hopefully be an owner of one for later in the year to be used for my own SS / Valve Power Amp' comparisons.   

As the owner of expensive amplifiers, both SS and tube, I challenge the premise of the question.  Amplifiers have little or no impact on sound stage in my experience.  Look to transducers (i.e. speakers, cartridges, if applicable), most importantly speaker placement (move them out from the back wall 2'-4'), add a couple of subs if at all possible. Room acoustics is always of paramount importance in every system.  These are the primary ingredients for good sound stage.  Very much secondary in importance until you get into very high, high end systems where they take on added significance would be your preamp and phono stage.  At this stage noise becomes a big focal point for improvement and it is here where grounding, wires and every detail becomes important.

Assuming good room acoustics and speaker setup and just focusing on the immediate question:
-Higher quality power supplies.

-Superior noise rejection.
-Better topologies to take advantage of and work in conjunction with the lower noise power supplies.

It is a complex relationship within the entire amplifier design from power socket to speaker terminals, but for sure a common factor is very low dynamic noise floor. If the microdetail is blocked by higher uncorrelated noise the sense of air and space (soundstage) is diminished accordingly. The very finest amps do not mask low level detail, but get out of the way of the upstream signal, which also should be on a similar performance level of the amp to get the most out of the system. 

I use a 2003 Yamaha RX-Z9 RECEIVER in "Pure Direct" mode through my Merman speakers and have not heard anything that has ever "soundstaged" and "imaged" as well as my system in my room..., and I’ve been to CES and T.H.E. Show.  This is just the two speakers (18"/10"/AMT), and there is no eq or room correction, either electronic or physical being used. 

The system brings the venue into the room rather than taking the room to the venue. And the equipment has the volume and headroom to bring ANY SIZE venue into the room.  Total investment is <$6K.

https://youtu.be/OmWNOi6b4ak

It seems to me that if an amplifier contributes to soundstage and imaging, it is through improved channel separation and crosstalk rejection.

.,,,The Room‼️and speaker placement….