Power Amplifiers

Mark Levinson Power Amplifiers: only 4 designed, one became a legend.

ML-2

Class A

ML-2 2 × 25W monoblocs (the legend).

Class AB

ML-11 2 × 50W class AB (the entry level)
ML-3   2 × 300W class AB (the huge one)
ML-9   2 × 100W class AB (the baby ML-3).

What does class A or class A/B means ?

To understand the difference between class A and class AB amplifiers one can read the according page on the Wikipedia.

Basically, class A amplifier need a lot of current from the power source even when idle, so the run very hot. Only one transistor will amplify the whole signal.

The class AB amplifiers do not have this drawback, they can get hot but not that much and they need only a small amount of current when idle. Class AB means that the amplifier runs in class A from 0 to X watts and then switch to class B. Of course you don't hear anything, it's just a small change in the electrical flow inside the circuit. Typically the value X is a few watts.
Some simple explanations with schematics on this page.