When a class A/B amp is run at low volumes, it works as a class A amp, which are the most accurate, but also the least efficient. When the A/B amp is run at high volumes, it operates as a class B amp. Class B amps produce more distortion than class A amps, but are also more effecient. Overall A/B amps are more accurate, or have less distortion than class D amps, but are also less effecient.
Class D amps are much more effecient, but the sound quality is much lower than that of A/B amps.