This is the biggest issue of the power amp section. The fixed diodes do not allow for adjustment of the idle current, the idle current is low and it is the crossover distortion that degrades the parameters.
By simply changing the diodes for a Vbe multiplier, we can adjust the idle current and get much much lower distortion especially at lower power. Unfortunately, Vbe multiplier circuits were not used back in 1966. Below we can see how it works with crossover distortion reduced by a properly set idle current.
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This would be a competitive amplifier, even nowadays. Removal of crossover distortion would remove the "transistor sound" of the sixties.