How Do Amps Affect Soundstage?

Mossy, I also have a 4B3 Bryston. I’ve been searching for a compatible Tube Pre that matches without blowing my load on an ARC Ref 6.  The pairing makes a ton of sense on paper.  Following with great vim and vigor. 

I used to have a hybrid pre with the Bryston that I could really crank, but was a combo that was very bright thru my Focal 1028be.

The mistake I have made is buying components piecemeal and not taking a system approach.

Good luck finding a fit

Tube amps change your frequency response. Heck ya that impacts sound stage. Some say distortion emphasizes some frequencies so my guess is that impacts sound stage too. Distortion I think impacts ambience which I used to confuse with sound stage. I think a lot of people confuse the two. In my own tests solid state amps all are similar for sound stage. I would assume ones meant to be like tubes would do the same as tube amps and sound different.  If I run tubes and solid state on the main system and let the Lyngdorf do it's thing they all sound similar. Not the same, but similar.

I highly recommend to anyone in this hobby that they commit part of their upgrade budget and go to an audio show (I've been to AXPONA, Florida, and Pacific Audio Fest). You will come away with many of your preconceptions broken or severely damaged. There are rooms with solid state amplification that have extraordinary imaging and rooms with tubes that have mediocre imaging. And vice-versa.

I'm a geezer who has subscribed to Stereophile and TAS for 40 some odd years and I have never seen the experts answer the OP's question. There are lots of theories, mainly from amp and speaker designers, who tout their unique assemblage of parts and circuit design. They each say that they know the secret of superior imaging but if any single approach was better it would have been largely adopted by the rest of the industry.

I have experienced what the OP has observed, except that in my case it was the difference between two SS amps. I'm driving Thiel CS6 speakers (86db with a brutal impedance curve) with a Krell KSA 300S amp. I took the amp in to get it recapped and I tried hooking up my Onkyo AV receiver up to my speakers for the interim. When I listened to the first cut I literally laughed out loud. Compared to my Krell the sound was like an AM radio. I realize that this is an extreme example but anyone who thinks all amplifiers sound the same hasn't heard a comparison like this.

I believe that imaging and soundstage are primarily impacted by the design and performance of the speakers,  their positioning within the room and relative to lhe listener, and the room acoustics. The contribution from the amplifier falls far down the list compared to these factors. As @othercrazycanuck  mentions, tube amps are more prone to variations in frequency response and higher distortion, which are more likely than SS amps to impact imaging and soundstage. I tend towards the school of trying to recreate the intent of the musicians and recording/mastering engineer, which suggests minimizing additions to the sound. There are as many opinions on this as there are audiophiles, however, and beauty (and soundstage) is literally in the ear (and brain) of the beholder. So if it sounds good to you...enjoy.