question of soundstage

Yep, lots of factors. 

The Sugden has greater channel separation compared to your tube amp.

Soundstage is a test you should use to evaluate an amp.  Cayin is not a high end tube amp and it isn't surprising for it to be outperformed.   My guess is that it is PC board based rather than point to point.  but that is speculation.

I'm not knowledgable on sugden, but they claim to be hand made in the UK for many years so that would point toward quality.

Jerry

I have Sugden Mono blocks, Before that I had the Sugden SPA-4 power amp. Same huge soundstage.I have a Sugden IA-4 in the basement,on my second system, same quality. Could be part of the Class A, or the synergy with the other components as others have stated.

I had a Krell FPB 600 hooked up to an ARC Ref 6 SE. I then upgraded my Krell to the ARC Ref 750s. Huge difference in sound stage. Soundstage definitely comes from the amp imo.