Actually difference between class A and AB?

Class A amp sounds like holy grail of HIFI world.
But class A amp are much heavier, more expensive and rarer than class AB or class B design. Few can afford 100 LB weight of a 2-channel 100w/c amp.

Is there any actual difference between class A and class AB when listening to actual music? And what is that difference?
The difference if fairly simple. Class A amps run at full power all the time. Class B amps run 'on demand'.

Class AB amps usually run in class A at lowest power, then switch to class B at higher power.

Whether one is better than the other is hard to say, largely it depends on the individual design.

Personally I use 30wpc Class A tubes and it, to me, is far superior to anything I've owned in an AB design. But that's just my opinion.
Yes, the A/A-B debate verges on the '3rd rail' of hifi. Toss in NFB and it is downright toxic.

However, would anyone consider it true that the best A amp is better than an inferior A/B design...and the reverse? Meiwan's 'individual design' remark is correct, IMO.
everyone is talking about design difference. No one is talking about difference when listening. Can you hear the difference!???
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.