Those symptoms indicate a very low power supply voltage and the only audio getting through are the highs (very low) passed by open stage capacitance.
The power supply is shutting down due a thermal runaway short in either channel (a short shuts down the power supply) or a power supply regulator problem. These types of problems can be analyzed by spraying suspected components with freeze mist.
We no longer do shop work because it is cheaper to throw it away and buy new. If you would want to pay shipping I would shop it out and refurbish for $125 plus parts and dont cherish it because the schematic would not be available. In the case of JBL Mackie and a few others this would be no problem. The only other option is finding an old tech like me and buying a bunch of BUD.