What is the difference between a Class A amp and a Class AA amp?
There is no such thing as a "AA" amp,that is,unless you've been to "Ming-DA" amp homepage !
The classes of amplification describe the operating bias and resulting duty cycle of the active devices in the circuit (Bipolar transistors, tubes, FETs, whatever).
They are classes A, AB, B, and C. There is no class AA in any engineering textbook. I suspect any reference to "class AA" may be some proprietary scheme of ultra-linear biasing, but that is not an accepted terminology or class.
Also, reference is sometimes made to "class D" amplifiers; these are amplifiers whose devices are actually operated in class C with some sort of high-frequency pulse modulation density scheme to provide a high-efficiency audio operation when the HF is filtered out.