1500 watts rms in a powered mixer format is hard to believe, but that’s what these things kick out: or in this case 750 watts, because one side of it didn’t work. There are still a lot of these about, and that’s getting on for twenty years since they first came out. They must be reliable, as there aren’t all that many find their way to the workshop.
A major power amp fault is usually a rebuild. These are bipolar amps using MJ15022 and MJ15023 transistors in a complementary design. If these go down, they’ll often go collector-emitter short, and blow back through the base which takes out the drivers and a fair bit of the bias chain and even before that. It’s safest to replace most of it and certainly the bias chain components, because if certain of those get fed up after the repair, it will knock the whole lot out again. You’ll also expect it to take out the switching rail power supplies with it.