I go through that every 5-6 years or so.
Not just with tone either...
Whole new rig.
Not just from one model to another either..its jump the brand ship and start over.
When I fist started and was Stupid...
1992-1996 Crate Stealth
1996-2005 Marshall ValveState 8200
2005-2014: B52 AT100 (l wish...
The Jubilees don't really go to 'output tube saturation'
The output section is similar to all the other Marshall designs, but the preamp doesn't drive the output section into saturation or any pleasing distortion.
hmm...how old is the amp build?
The caps shouldn't be bad...not to say you couldn't have gotten a faulty one.
If tubes didn't solve the problem..
It really displays 'bad connection somewhere' behavior.
When i finished up my amp build...I had what you describe...SOMETIMES.
Hell haseth frozen over.
I picked up the Headrush Gig Board.
Primarily to use as an effects unit through the effects loop of my JVM.
Just the effects.
They sound decent enough, the MIDI allows me to change JVM channels/modes. And I can get the same functionality as I did with my Rack...