I’ve never had a Dragon, but on a Cobra you have to keep the channel volumes high, or dimed, otherwise the amp will sound fizzy and weak. Probably not your problem, but worth mentioning regardless.
I have a dragon. have not pulled it out in a bit, but the way the channel volumes function in relation to the master is different than what you normally run across.
it's tubes most likely unless you are running something odd with the controls.
but I would do nothing to the amp but replace the tubes and see what happens. any more than that will fuck up a potential return.