The 'Bias transistor' should be close coupled to the output transistors to achieve reasonable tracking. The drivers are not so important as when they warm up, the quiescent current increases as does the output transistors heat, that warms up the 'Bias transistor' which reduces the quiescent current. Helping to keep the amplifier more stable.
I did a lot of this kind of work in my time. The output transistors get very hot at 4 ohms and no matter how big the cooler is connected, it was not enough. Does this bias transistor offset this heat phenomenon?
If the heat sink isn’t big enough it isn’t big enough. The bias regulator helps keep the quiescent bias from running away. It just needs to track the hottest transistor, which is typically the output. But it can do nothing about the dissipation and heat due to loading. That’s just a matter of supply watts minus speaker output watts, which can be high at 4 ohms at war volume.
