I have fixed it now, thanks.
Couldn't think what else to try, so I fitted a spare tube, cooked it overnight with heater only, now I'm getting 1.7kW out with my max. 70W? available drive.
It's perfectly stable and docile, just like a triode amp normally is.
Can't imagine what the fault inside the first tube is. I've written "NBG" all over it so it never wastes my (or anyone else's) time again.
