yeah, its most likely tube failure.
And dude, don't be afraid to go in there. You won't fuck anything up, unless you do it when you're drunk and have no concern for the value or condition of your equipment. To do the microphonics thing, turn your amp on and set it as you normally would, except turn the volume control up a little more than what you'd use it at for bedroom/household levels. For some reason I'm thinking that I read that you have to set all controls to 12'o clock but I'm not sure; I would at least max out the gain, though, to help you get more response from the test.
Anyway, just do that, and then take a pencil and just poke at each preamp tube with the eraser. I like to kind of lightly flick/tap at the tube from an angle, rather than prodding it straight on, but I suppose it doesn't matter. If you hear any feedback-type noise come out from hitting a tube, then you know it's bad. You'll be able to hear the tapping against the tube coming through the speakers; this is normal. Its just that if any sustained noise rings out, then you know that you've got a bad tube.
Do this for the power amp tubes, just to be safe. When these go microphonic, though, you'll more often get a buzzy ringing noise at certain notes, rather than a sustained feedback squeel. But if you hear anything rattle inside the tube when you tap it, that also means its bad.
How old are your tubes anyway? Or how long have you had the amp? You should generally replace tubes maybe once every 2 years, depending on how often you play the amp and how hard, like if you practice with a band and play shows with any frequency; even if the tubes are still 'good' and in perfect working condition, they do lose their tonal sharpness and vibrance after a while.
There's nothing sweeter than putting in a new set of your favorite tubes and letting 'er rip....
