#3
Basically, in a Class A amp you've got the power tubes running all the time, both on the positive and negative half of the wave. In Class AB, you've got one power tube (or pair of tubes) running half of the time, amplifying only the postive half, and the other power tubes amplifying only the negative half. AB amps are more efficient, so you end up with more wattage out of the same number of tubes.