The advantage of the AB design is that the crossover point from A to B is a smooth change that we are not supposed to notice. I am not a engineer but my understanding is that at some point the two amp classes share the duty so that there is not an audible changeover. Class A amps are very linear but very inefficient so they are good for low power uses. Class B amps don't due low power very well as they take a definite value of input signal before they begin working. However, they are more efficient than class A amps but not as linear. They are good for high power uses. Combine the two and you get the best of both worlds - Linear low power operations and High power capability.