What could I expect to hear from a Class A amp?

My Sugden compared to many other fine A/B amps I've owned is much quicker, open with very solid bass. Vocals and acoustic piano and bass are particularly well delivered.

As mentioned above, it is probably more about overall design and execution than choosing pure Class A vs. Class A/B.

That being said, all my current amps are pure Class A:

Atma-Sphere MA-1 mono blocks, Audio Mirror 45 SET mono blocks, and a couple of different Nelson Pass DIY amps are all pure Class A amps.

What do they all have in common? Very liquid and organic sound that you can listen to for hours and hours (as well as significantly higher electricity bills).

George

Well designed class AB doesn't have crossover distortions because it is removed by negative feedback. There is no THD or IMD but instead hidden TIM distortion. TIM causes odd harmonics (sharp sound) and fatigue (brain fills moments of silence caused by momentary saturation).

Class A doesn't need feedback as deep because of operation on linear characteristic of transistors but also because it needs less of overall gain before output stage.

Unfortunately while class AB consumes, playing music, on average only few percent of maximum rated power the same time class A consumes full rated power.
Efficiency is extremely poor - in order of 12%.

My 2x200W class D amp consumes, during playback at full power about 35W while class A would consume 3200W (even idling).

Bearotti - class D is very good at low levels (lows, highs, definition, dynamics, clarity etc)

Class A means that the bias on the 'device' (transistor or tube) is set to allow the device to be "on" 100% of the time. Very linear for the signal (that's why they sound good), but poor efficency. That's why real Class A amps are huge and expensive.

Class B amps are used in a "push-pull" arrangement - two tubes or transistors are used. While the signal is positive, only one tube is 'on' while the other is 'off'. Visa versa for the neg signal. Not used in transistor
audophile amps because of crossover distortion.

Crossover distortion is caused by the fact that the input of a transistor needs 0.7 volts to forward bias the junction. By keeping a small dc voltage at the junction to prevent the signal itself from being used to turn on the junction, this eliminates crossover distortion.

Class AB is a compromise between pure class A (sounds great, but runs very hot and is expensive) and class B (which runs cooler - more efficent - but doesn't quite sound as great as class A).

I don't believe that negative feedback is used to eliminate crossover distortion, but rather "regular" or non-linear distortion in general. Feeding back part of the signal in a 'negative' or degenerative manner increases amplifier linearity somewhat, but can produce other kinds of distortion (intermodulation, odd-order harmonic distortion, etc.)

Class C is for RF amplifiers almost exclusively (except bullhorns)