Hi Tim,
Lawrence is perfectly correct, but his comments might strike you as a bit odd! To have the output valves running at maximum dissipation all the time, even with the volume at minimum seems counter- intuitive!! You only expect things to get hot when you are creating a lot of racket
The output stage works in 'class A' mode. This means the valves are biassed to work on a nice linear portion of their transfer curve, so even with no signal, they are both conducting a constant current near to their maximum limits.
That's why you get a steady voltage on the cathodes sufficient to power the preamp valve heaters.
Class A gets round some of the flaws of push pull output stages and gives really good quality sound but is very inefficient: that's the down side of it. On the plus side, it will keep your room warm!!
Hope that answers a question that you may not have considered yet!
Right, time to do a bit of circuit tracing.
All the best
Nick