The power amp provides the power supply for the preamp. There is also a power-up mute signal to the preamp. otherwise the power amp has nothing to do with the preamp, and certainly not individual channels.
You have the function listing of the ribbon cable to the preamp. All you see there is power supply, the mute, and L and R signals. No other circuits.
If the amp is still getting hot, look into the power amp. Remove any shorted power transistors from the board. I find it easier to snip their legs off near their body. Toss the old part. Now it is easy to remove the remaining leg bits from the solder holes. That alone should stop a lot of burning.
The high voltage wires from the transformer just plug onto fast connectors at the end of the board. You could just pull them, but I think we'd have to fudge the mute, since it refers to V-.
As I mentioned, IC6 is just a 4580 dual op amp in a SOIC package. I'd get them at Mouser.
There are five little ICs along the edge of the board on the bottom side. From the power supply end they are: IC6,5,8 and then 7,4. The full size ones - 6,5,4 - are all 4580. The two smaller ones are the limter chips, 3080 type.