The model 7700 is a traditional class A/B liner design. This type will yield better sound quality than any of the other classes except for class "A" types which are very rare and expensive (and extremely inefficient).

Class "G" is a rail switching type which uses a lower power supply rail up to a pre-determined output, then switches to a higher voltage set of rails. This switching is never perfect, but the point of switching is usually placed at a high enough level that is is inaudible over the signal.

Class "D" amplifiers are digital - technically they use pulse width modulation. They are very efficient, but there are several problems with this type of amplifier, not the least of which is a high amount of ultrasonic clocking signal which cannot be filtered from the output.

The Outlaw amps are extremely well made - Buying the 7700 is a real no-brainer of a choice.