Nope - no schematic Fernando.
I've subscribed to the Marshall forum so there's now at least three of us there with an issue. Not good for Marshall is it?
Indeed not good.
From what gwEm says, sounds like it is a well-known problem with this sort technology, which would suggest that Marshall don't understand it fully.
I've logged into the "super secret" Marshall schematic service facility and there's no schematic there. I expect there won't be until it goes out of warranty.
Regarding the "Power Scaling", what I have gleaned is that the screen grid voltage and bias voltages are varied, rather than the plate, screen and bias voltages, and that this is controlled by some digital which has a number of stepped power levels, and this seems to be where the problem is.
As ever, using "cutting edge" approaches often reveals unexpected problems.