Got a related anecdote, just for kicks...
I while back there was a thread in the Modern Era section in which a guy had fired up his DR without a speaker connected, and fried at least a fuse and maybe more... I piped in rather unhelpfully that I'd made the same move with my JCM800 recently, but with no apparent effect on the amp, it still purrs along like nothing happened.
This reminded me of another thread (which I can't find) in which some other poor bloke had left his Mesa on standby for a few days, and flipped the amp on to find that it was DOA (probably just another blown fuse, I think/hope).
Not long after that (last weekend, in fact) I went on a 4-day trip out of town, very relaxing, aaahhhhhh.... came home, unpacked, went to work the next day; came home, looking forward to sneaking in a few minutes of playing time in my den; flipped on my LSC, played for a while; and noticed that my JCM800 was on frickin' standby.
Thinking, oh crap, nice move especially after having already just put it through that empty speaker load session. I tried to remember when I'd turned the thing on... I hadn't been in the room for 5 days at least, probably turned it on the last time I'd been in there -- the amp had been on standby for a week or so. Dumb ***.
I tentatively plugged in, noticing how nice and warm the chassis was, oh crap oh crap oh crap... turned it on... and good gracious, the thing sounded like Angus Young, just like it always does. I've played through it again over the last couple of days -- not a problem. It sounds great.
I'm a serious idiot, but that JCM800 is a really sweet piece of work.