Ralph, I hate to disagree, but a pure Class B amp (like the Quad 405), switches from the upper set of transistors to the lower set with no region where both are operating. Class AB has an intermediate region where both are operating.
In a typical Class AB amp, whether tube or transistor, there are three operating regions: the plus direction, which activates the upper output transistors (or tubes), the middle zero-crossing region, which activates all devices at once, and a minus region, where only the lower devices are operating.
The size of the middle, zero-crossing region is at the discretion of the designer. If this middle region is so large that the B regions are never activated, it becomes a Class A amplifier (by default).
The Quad 405 had no A region, and relied on the feedforward system to supply current and voltage for +/- 0.7V region where all output transistors were turned off. As a result, it ran quite cold, but if you had a good enough distortion analyzer, you could see the switching region along with a spray of harmonics.
Class C is reserved for radio frequency transmitters ONLY. This has massive distortion since not all of the waveform is amplified ... there’s holes in it. It doesn’t matter in RF applications because tuned circuits filter out all of the harmonics, and Class C is more efficient than Class B or AB.
Class D is a switching amplifier, akin to a switching power supply. Pulse-width modulation converts the incoming analog signal into PWM, which is applied to very high speed switching transistors. A lowpass filter at the output removes most, but not all, of the ultrasonic grunge. It is normal to apply substantial feedback to linearize the PWM modulator and correct for small timing errors in the switching devices. (In PWM, timing errors translate into distortion when lowpass filtered.)
Here's a succinct explanation on Wikipedia: