I play in several school bands (jazz, rock and fusion) and play through the school amp, which is a Fender Bassman 250/210 combo with an external 15" cab attached. While jamming last night it started farting and distorting horribly so I assumed it was a blown speaker, but that's weird because I only had the amp running at about 25% volume and the clip indicator wasn't flashing. Any ideas as to what might have caused this?