[mulveling]

In the most simple terms, from what I've gathered over time:

Class B - Is quite efficient in that it doesn't generate a lot of extra heat, but it will sound horiffic due to the types of distortion inherent to this amp topology

Class A - Very inefficient, generates gobs of heat even (especially) when the sound level is low, but inherently superior to class B topology in distortion and sound quality.

Class AB - Kind of a mix between A and B. Can be designed to lean more towards one end of the spectrum or the other. Much much better than class B. It's arguable whether a well-designed class AB amp gives up anything to a good class A design. Some amp designs run the first several watts in class A (first 25 Watts for my Parasound JC1), then automatically switch to class AB when higher power levels are needed - I guess you could call these "high bias class A/AB".

You won't find class B amps on the consumer or hifi audio markets - it's not appropriate for these uses. The vast majority of amps on the market, tube and solid state, are some form of AB. The pure class A amps on the market are typically very heavy, very big, very expensive, run very hot, and often have relatively low power ratings.

On the issue of distortion and SQ, of course there's more factors to it than just the topology chosen. Many tube and/or class A amps have higher published distortion specs than competing class AB designs. This is often the result of the higher amount of global negative feedback typically used in the solid-state AB designs. The recommended amounts and effects of this kind of feedback can be a hotly contested issue, but most would agree that lots of it is bad...it seems to make amps look better on paper but sound worse in reality. Common complaints are that it makes the sound harsh, hard, and/or fatiguing. Some amp designers have such a low opinion of it that they design and advertise their amps as "zero negative feedback".

Preamps can also be built along these various topologies, but it's much easier to build a class A preamp due to the reduced power levels, so many hifi preamps are class A.

Also, opamps/IC's in the signal path really aren't capable of running in pure class A, so an amp or preamp that uses them probably isn't pure class A. The class A preamps use either discrete transistors or tubes, the class A SS amps must use discrete transistors in conjunction with huge heatsinks, and the class A tube amps are either flea-powered (3-7 Watts) or behemoths.