So, this happened.
I had my rig open yesterday, with nothing plugged into the input, because I had a microphonic preamp tube. I replaced the culprit tube, but when I turned the standby switch off to see if it had worked, the amp started squealing like a stuck pig. just a high feedback-esque sound. plugging a cable into the input jack caused the sound to go away, even when the cable wasn't plugged into a guitar. About a month ago I replaced the input jack with a different one, on the grounds that the original's plastic casing was falling apart, but wired it the same way. Mono input, with the same 1M resistor across the hot and the ground as the old jack, I just moved it over. I tested the resistor with my meter, and it read 1M ohms, so the resistor isn't open.
What in the hell could be causing this? I thought it was a tube issue (and, I suppose it might be, but that just doesn't make any sense to me as to why the amp would function normally with a cable plugged in, but with no cable plugged in, it goes all haywire.
Just to get the obvious out of the way, has Drew been anywhere near your rig?