Weak, distorted sound - 1 of 4 green LEDs on power amp not lit up

This speaker (a refurb bought from Guitar Center) belongs to a friend, who said it was putting out sound, but the output was very weak and distorted. I pulled the power amp module out of the back. The heat sink was fairly warm. Surprising - since it was switched on for a few hours, but had no signal running to it. The fan was also running every 10 - 15 seconds for a few seconds.

On the power amp board, I noted 3 green LEDs lit up. A closer look indicated there is a 4th LED - not lit. My suspicion is that these LEDs indicate something about the status of the power transistors, and that the unlit one indicates one of the transistors (or a part of the power amplification circuitry) is not functioning. Any suggestions on what to test/try? Should I pull off the heat sink and look for the black spot on the circuit board for the remains of the fried component?

Also - I did not see any reference to these LEDs in the literature available online. (i guess I'm not supposed to remove those 6 black screws?) Some info on the meaning of these LED in reference material would be useful.
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