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.