Wonder no more, there are many others with many different brand of amps that do this. Obviously it needs repair unlless you have a problem with the cable or source you are using or have forgot to turn up the gain controls which I am sure you have checked. There are so many possibilities that there is no way we can say open it and replace this... If you get a small pop in the speaker when you turn it off, then the power portion of the amp is probably OK. A common probllem on amps that have a preamp out or power amp input jacks is a failure of the switch contacts on the crummy jacks that pass the audio through from teh preamp to the power amp...