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.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner