Everything is a system, so don't focus on one part and ignore the rest.
Spend a day working on it. Set up the system in your garage or somewhere. First, power up the mixer with no speakers connected, and no speaker cords. Let it sit running. Does it get hot that way? If so, it may be underbiased.
Plug a speaker cord into one channel out, and at the free end of the cord measure for any DC voltage between the tip and sleeve of the plug. SHOuld be none. Now same for other channel out.
Now connect speakers to one side. I don;t know the unit, it is stereo? Let it sit running. Does it now start to get hot? Then the other side, then both sides. No sound yet, just running idle.
If the thing idles cool enough, then we start running signal througn it, first pan all the way left and run a good signal through only the left speaker. Does it get hot? Now change over to only the right channel speaker. Same, does it get hot? What we want to do is isolate the problem.
You may have checked your wires, but a hand meter won;t always detect an intermittant problem. More important, what if one of yor speakers has a problem with its crossover? SO running just one speaker at a time may help us identify that the problem only occurs when one of the speakers is in use, but not the other. We have to either blame or rule out the speaker cabs themselves.