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Class B is high distortion and A is more linear but low efficiency so deep AB means more like Class A with a higher minimum current to improve linearity.

But the method employed seems to also match the impedance of harmonics to improve stability factor and PAE while reducing input levels with a deep AB front-end with 2 other stages to increase Pout > , PAE  and lower distortion with > 10 dB gain.  Bravo Iranian students.