question of soundstage

Which speakers are you using?  It may be a question of synergy.

. . . also not all tube amps (or tubes) are created equal.  As far as sound stage, I remember hearing a long time ago that monoblocks excelled at that.

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