So, I fire up the rig for practice last night, and get this sound that's sort of like what I'd imagine it sounds like inside a jet engine as a plane is about to take off. At first I suspected the power amp, and plugged the TA right into the board (the drummer is in a separate room, so we send him a mix). Same thing. I got through practice by playing through the singer's old pedalboard into my power amp.
I'm pretty sure it's just a wonky tube, I hope. I've got a box of worn-out (but not microphonic) tubes, so I'll try replacing them one at a time, to see if I can find the offending tube.
That oooozes bad tube