Subwoofer caught fire while on standby?!!

[Samsang]

Probably a filter capacitor in the sub's power supply. A common failure mode for high value electrolytic capacitors is an internal short circuit. The high current through the short makes them blow up with a loud "POP!!!

Mind you though, by "common failure mode", I mean common among those that fail. Electrolytic caps are amazingly rugged and reliable, with a very low failure rate. There's no reason to expect that this will happen to others out and about, so quell your fears and resume normal normal routine out there.

It's possible that the cap didn't take any other components with it when it popped. If so, it will be a quick and inexpensive repair. Don't expect that, necessarily, but it could happen. Good Luck!