Douglas Self stated that increasing bias often leads to increased distortions. He also measured it. The reason for that is changing transconductance. Transconductance is output current to driving voltage gain, that is rapidly changing when two transistor current (biased region) changes into one transistor current (outside of biased region). He stated:
The main reason of going into class A is to reduce feedback. Once feedback is set damage is already done (TIM distortions) and playing with bias won’t change it. Perhaps there is a way of compensating for the change in transconductance (gm doubling) or adjusting feedback dynamically but it is very complicated. My Benchmark AHB2, class AB amplifier, uses separate "error output stage" to avoid recursive feedback and TIM distortions (AAA patent).
It is not generally appreciated that moving into Class-AB, by increasing the quiescent current, does NOT simply trade efficiency for linearity. If the output power is above the level at which Class-A operation can be sustained, THD increases as the bias advances into AB operation. This is due to so-called "gm-doubling" (ie the voltage-gain increase caused by both devices conducting simultaneously in the centre of the output-voltage range, in the Class-A region) putting edges into the distortion residual that generate high-order harmonics much as under-biasing does. This vital fact is little known, presumably because gm-doubling distortion is at a relatively low level and is obscured in most amplifiers by other distortions.You can find it here (5.3): http://www.douglas-self.com/ampins/dipa/dipa.htm
The main reason of going into class A is to reduce feedback. Once feedback is set damage is already done (TIM distortions) and playing with bias won’t change it. Perhaps there is a way of compensating for the change in transconductance (gm doubling) or adjusting feedback dynamically but it is very complicated. My Benchmark AHB2, class AB amplifier, uses separate "error output stage" to avoid recursive feedback and TIM distortions (AAA patent).