Amp broken?

Yesterday I was preparing to sit down and have a listen on my tube pre/amp so I turned them all on and let them sit
for about an hour to warm up.

The source CDP was on but I hadn't hit play.
I dozed off for a bit and was awakened by the sound of one of my speaker woofers flapping uncontrollably!
Naturally I jumped up and turned everything off as quickly as possible!

It was similar to a sine wave or something like that being played.

I thought it was very odd because I hadn't changed my setup in a few months and things have been working perfectly for all that time.

I decided to unplug the preamp and cdp and swap the speaker terminals to see if I could narrow down the problem to something upstream or the amp or speakers themselves.

I also took off the shroud covering the amp to see if any of the tubes weren't working.

I started the amp up and after the 2 second warmup the other speaker started to flap (and I also noticed the tweeter was going also, the first time I missed it because I was in shock I think!) Also all the tubes looked like
they were lighting up.

Does my problem ring any bells as to what might be the culprit here?

Should I bother testing anything else? I had already unplugged the unit and let it sit for a considerable time before I did my second test!