[Grubhead]

The most obvious cause of a power transistor overheating if it's not itself, would be the power supply circuit that feeds it. Voltage measurements should track down the cause.
Are the transistors overheating on both channels or just one?
If it is just one, test and compare the voltages of the good channel with the bad one.
If both channels are overheating then you have a power supply problem, or something that is common to both channels.