Solid state amplifiers and sound stage, especially front to back "depth"

I never really thought the type of amplifier would affect soundstage. Speaker position, reflections,  the recording I would think has more to do with soundstage. The one thing that isn't controlled by those is SPL at listening position. See if increasing the volume helps. Is that SET amp low watt? It might clip before it gets as loud as the other amps. Other than that I have no idea why. 

@djones51 So far, the consensus is that amp type affects soundstage a lot. So far, if I'm paying attention, only the Sunfire 300 amp has been given props for an amazing soundstage.
The SET 400 is 225 wpc into 8 ohms.

Have you ever tried any tests like this? I don't know about soundstage but it might be of some value.

https://www.audiocheck.net/audiotests_ledr.php

Speaker position, reflections, the recording I would think has more to do with soundstage.

djones51, I absolutely agree. Also the design of the speakers. I spent many listening sessions adjusting speaker position to get the optimal imaging. Toe-in and rake-angle helped fine tune the soundstage. Using absorbion or diffusion on the front wall between speakers can tighten focus. Furniture or a large rack between the speakers can reduce depth. In my room, which is a living/listening space, I discovered the room was too damped and needed some areas of reflection.

But ultimately, I think the amp and preamp have to be capable of presenting a wide and deep soundstage.

Regarding an amp or any other electronic gear upstream’s relative ability to do imaging, it’s generally about achieving low distortion and detail retrieval. That is needed to best deliver the spatial cues that enable imaging captured in a recording completely. Then how the recording is made is the other in big factor.
Then of course the speakers setup and room acoustics determine exactly how those cues are rendered and imaging and soundstage actually produced.
The best soundstage depth Ive ever heard by a huge margin was an all mbl system with SS amplification set up in a highly customized showroom. With master recording R2R as a source you could easily identify exact location of individual orchestra instruments both front to back and side to side in a large tapered rectangular area. That was at now defunct United Home Audio in Annapolis Junction, MD several years back. Nothing else I’ve ever heard comes close on regards to precise 3-d soundstage and imaging depth. Totally holographic. Same setup by same vendor was just OK at local shows in more conventional and less optimized room setups.

In my system, holographic soundstage and imaging took a big leap forward when I introduced the Bel Canto ref1000m Class D amps to my Ohm Walsh speakers. It was totally disorienting at first until my ears latched on to what was happening. Night and day from prior Class A Musical Fidelity SS amp.