From the research I've done, it seems most likely to be the STK404 that's the problem and it's the component that most commonly fails. I want to run this by some of you and see if anyone has any other ideas before I try replacing it. The problem started with the amp taking a couple of minutes to "warm up" before sound would come through. After a couple of weeks, it stopped playing sound altogether. The preamp doesn't seem to be the culprit. The clip LED lights if I turn the gain up and play a note. The XLR direct out also transmits the signal. Neither the internal speaker or an external speaker works, so it seems to be a problem within the power amp. My hesitation comes from this: I tried testing the STK404 with a multimeter as stated in other posts. It says there will be a short (continuity) between pins 11 & 12 and 10 & 13 if the STK404 is dead. I did not get that short. It measured high resistance as it should. I also tested all of the large capacitors with the multimeter and none seem defective. Is there anything else I should test or look at before jumping into replacing the STK404?