Re: Bad Speaker. Anyway to tell just what the problem is?
Try pushing gently on the cone (with the amp off). Can you hear the voice coil rubbing? There are a bunch of reasons a voice coil would rub. A mechanical shock (dropping the cab) could knock the magnet out of line. Or, too much power could heat up the voice coil, and melt the glue holding the whole coil/cone together, allowing it to ooze out of alignment.
No matter what the cause, it will probably need reconing.