I am so sorry to hear this. That Rotel is a real marvel.

Given that two left channels are gone, I'd say it's fuses or problems with a couple of the windings from one of the two massive toroid in that beast. Why it might be fuses or windings would be a good question. There's thermal and overload protection in the unit also which should prevent fuses or windings from opening. Maybe the thermal or overload protection for those two channels is pooched.

You can lift the cover and look for charred bits. See where the windings terminate and measure their output voltages. If windings are gone, that's a new toroid. Voltages in there are hazardous by the way so you better know how to work around them.

I'd definitely try to salvage that beast.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated