The description points to all the things mentioned. It could also indicate problems with the bias supply and/or a bad filter cap in the power supply. Don't mess with it unless you know your way around.
 
If the output tubes are glowing orange anywhere other than the internal filaments then stop right away because you will need a tech and messing around could damage the amp.
 
A couple non-lethal tests would be start the amp with the power tubes removed. IF the hum remains, that points to the power supply or the filament supply. If the hum stops then it's probably the output tubes. To double check you can install the power tubes and remove the small tubes and start it up again. If it's quiet, then the problem is upstream from the power section. Try putting the tubes back in one at a time and re-testing. When the noise returns then you have a tube to change.
 
Now these are easy tests and can help an experienced guy locate the trouble. You have to power off between every tube removal or installation. They get hot. If anything I described sounds confusing then you should have a tech look at it.