Sounds almost like you have some ultrasonic oscillations. They'll make an OPT get super hot. But, I guess that's really unlikely, as that amp has grid stopper resistors thicker than flies everywhere, and if was an ultrasonic oscillation that bad the plates would probably be glowing red. Mant times, an ultrasonic oscillation will modulate other frequencies, and you'll have a weird audible hum along with it, or even other oscillations at audible frequencies.
Thermionic