To prevent thermal runaway -
In place of R5 try a low-power npn transistor like a BC109 with collector coupled back to its own base via a slider on a 1K-ohm preset potentiometer. Thermally couple this transistor to the output transistors if possible.
1.4 volts between collector and emitter. 0.6 volts between base and emitter as set by the 1K preset pot.