Check the HT is correct, the contact cooled rectifiers sometimes fail. I had a Bush with only 90v HT due to the rectifier it still worked with no distortion but the volume was fairly low.
Check the HT on the smoothing capacitor if it's OK there check the voltage on the anode pin of the valve is the valve getting hot as normal?
As you say there isn't much to it.. can't be too Major...
Rich