Why are one channel's transistors in a power amp getting much hotter than the other's?
See the resistors I've marked with red boxes: -



With no signal at the inputs, use a DVM (on DC millivolts) and measure the voltage across each of R721 and R621 - they should be about the same - maybe 5 mV to 20 mV. If the hot channel has a significantly larger voltage then try adjusting its associated bias current trimpot (the other red boxes) to reduce it to the "good" channel.

I think R721 and R621 might be these 0,24 Ω resistors marked in green but double check the PCB legends: -



Also up for consideration - do the transistors get hot when the speakers are disconnected, connected, or both? If they only get hot when the speakers are connected then it might be that C709 (the output DC block capacitor) has gone low resistance.