If it was there all the time and only became audible at high volume then I would say it is just the way it is, perfectly normal run of the mill noise, and you can track it down if you think it is worth it. But "occasionally" and "fine for a bit" then reappears indicates to me this is heat or power related. Could be a tube, could be something else, but more than likely not right.
Since you run mono then if it is both then probably not the amps, but more likely the preamp.
Since you run mono then if it is both then probably not the amps, but more likely the preamp.