So you have replaced yet another transistor then, since the ones you did yesterday when you got 400V, yes?
Well lets hope so then if you think that was the cause, still be inclined to replace the OP amps incase they have been damaged by the strange and abnormal events that the PSU has been having.
To answer your question about what if, if everything is replaced and there is no improvement then, we shall need other peoples involvement on the remaining circuitry as that is way out of my comfort zone then. What I would do if it was mine is to carefully and methodically lift a leg of each discrete component on each board and check it for value and ESR etc and if found to be OK resolder the leg and move on to the next part until I had covered every discrete part on the boards and then evaluate again.
Unless of course the service manual says otherwise, if it shows TP and voltages and trace patterns that should be present, then I'd check for them as stated in the manual. Its a long slow process of test and replace anything that is miles out of spec. I know of no other way unless I had a parts mule, in which case swapping out suspect cards one at a time is an option, but problem, is of course, you have no idea if the other cards are in working order til you try them.