Tom says,

The ideal duty cycle is 50%, with half time totally off and half
time
in saturation.

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But 50% off and 50% on corresponds to Class-B bias conditions.

This is inconsistent with "tons of negative bias".

All that's required is a plate load which allows a square-wave plate
voltage waveform during the "on" period of the operating cycle. (A
very wide bandpass filter circuit configuration sounds appropriate).

Subject to this condition, a less than 50-50% operating cycle probably
does provide, somwhere, maximum efficiency.

I like your lumping together of Class-C, F, X, Y amplifiers.
----
Reg.