Actually difference between class A and AB?

As always it depends on which two amps you are talking about, not just what class they run in. I have had several Class A amps and countless AB amps; overall I think the Class A amps sound better, but I am not using one right now. Have one in mind I may try but I like my CJ 350. Not quite as sweet in the top end but close, with more bass than most A's outside of Krell and runs a lot cooler. Just one aspect of amplifier performance to consider. Bottom line, get what you think sounds right.

The amp that sounds better to YOU is better, regardless of 'class'.

As Stanwal aludes, 'a' amps run HOT. If you can lift a 100x2 'a' amp, you are either real strong or don't buy it.

All things being equal, you'd have trouble telling the best of 'em apart. until you get the electric bill.

With a Plinius, which can be switched from A to A/B, A sounds more natural. Can't identify what crossover distortion sounds like because my ears are just not trained and tuned for that and most distortions tend to sound like any other frequency anomaly. I have heard some very convincing A/B amps, most of which I couldn't afford at the time.

No one is talking about the listening difference because the differences are so small with the latest crop of designs that most won't hear a difference , in fact many prefer the added dynamic's of A/B design's . Levinson was the first to shed itself of the class A circuit's , the 20.5 was the last I believe , the hundred pound hundred watt mono's ran as hot as large tube amp's . Even there reference amps of the late 90's were A/B designs . Class A amps were superior sonically 10 years ago or so but like so many things in audio the two designs have nearly reached convergence . I have a few friends that made up there minds years ago that class A is superior and won't change no mater what they hear .

The measurable difference between class A and AB is the crossover point. ie. In class A designs one tube/transistor amplifies the complete waveform. So bias current in transistor designs are at max levels all of the time. In a class B circuit, two transistors or tubes push/pull together to make the complete waveform. One does the (+) side and one does the (-) side of the waveform. That allows the bias current to be nearly zero at idle. The issue is the noise or not so perfect handoff of one transistor to the other as the waveform switches sides. One a scope, it can be seen as a discontinuity in the waveform at the zero line. Of course, todays amplifiers all but eliminate that crossover discontinuity. Older A/B amps had a brittle sound, an unpleasant edginess to my ears. I didn't like listening to them at very loud levels for long.