Class AB amps use "2 sets" of transistors where one set amplifies the positive signal swing and one set the negative signal swing. There is a point near the centre of the signal swing where both sets amplify at once causing some distortion. With Class A amplifiers the transisitors are "ON" all of the time and amplify the entire signal. Because of this Class A amplifiers are generally less efficient than Class AB. Krell uses "Plateau Biasing" which sets the level of transistor bias or "on current" to the proper level just prior to the signal arriving thus being efficient while at the same time utilizing the sonice benefits of Class A operation.
