Three weeks ago I bought a brand new Mordaunt Short 309 sub to replace the cheapo sub that came with my Yamaha AV100 surround package, but a few days ago I started developing an intermittent fault (the green power light would come on but I wouldn't get any sound from it). At first I thought it was a problem with my amp, however I have since connected the sub directly to the line out of the sound card on my PC and played some 30-200Hz test tones and it still doesn't make a sound. Although it started working again a few times it now seems to be permanent now. Obviously I will be taking it back to the shop I bought it from but is there anything I could have done to cause its demise? The volume was only set to 50% and even if it was set to 100% I'd hope that there'd be some inbuilt circuitry to prevent it toasting itself.