Actually difference between class A and AB?

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.

I recently compared 3 amplifiers, 1 Stereo, and 2 pairs of Monoblocks. The Stereo and 1 monoblock pair were reportedly standard class AB, with the stereo amp claiming "high-bias".
The 2nd Monoblock pair was reported to run class A up to a point.

The "class A/AB" monoblocks ran much hotter than the other amps, no surprise there.

In my case, I felt that at low listening levels the ClassA amp was in fact superior. I fell into using the term "inner detail". Whether that's the correct term from the Audiophile dictionary or not, I have no idea. I was absolutely convinced I could here more detail with the Class A/AB amps. I considered those monoblocks superior to both AB amps. At louder listening I felt this difference disappeared.

I consider myself NO golden ear and I don't subscribe to the tweek of the week, so the above is simply my 2cents worth on the subject.

I assume your question is directed primarily to SS amps, maybe including hybrids.

As also indicated by Stanwal, IMO the answer is; “it depends on the amp.”

There are excellent sounding Class AB amps and crappy sounding Class A amps. There are so many other factors involved such as the design, quality and size of the power supply; mosfet or bipolar output devices; the number, quality, and matching of the output devices; the use of negative feedback (although some respected designers use local negative feedback); input and output impedance as related to partnering equipment and speakers; power and ability to “double down;” and other critical design factors. As an example, the Belles 350A Reference amp is said to be a very good sounding amp by virtually every professional reviewer and most owners/listeners who have posted here - and that amp runs in Class B!

Class A amplification is not for everyone, no matter how good they sound. Class A amps are typically heavy, run very hot, use more energy, should not be left powered up all the time (as you can with cooler running Class AB amps) due to wear on internal parts from heat, and are generally more expensive to purchase per watt of power.

OTOH, if you have the ability to buy (and lift :>) quality Class A amplification, and you can deal with the issues listed above, there are some very good choices that many consider to be among the best in SS amplification. In my own experience with Class A, I have owned the Lamm M1.1 hybrids and two different Clayton models, each of which sounded very good to excellent. However, I need lots of power to achieve the level of dynamics I want from my speakers so the Lamm amps (at 100 wpc) would run out of gas at a certain level (although they sounded very good up to that point). My current Clayton M300 monoblocks with the most recent power supply upgrade are the best sounding amps I have owned. They do not run out of gas and offer 300wpc @ 8ohms and 600wpc @ 4ohms, all fully in Class A (when they are switched to high bias). They have a convenient high/low bias switch that can be set to low (200 wpc in Class A) for casual listening – and still sound very good. Other Class A amps reported to sound very good to excellent include the Pass XA.5 series of amps (30 to 200 wpc in Class A), Boulder’s top amps, Lamm M1.2 Reference and 2.2, Krell class A amps, and more I have not listed. At the lower/mid price ranges, many have reported the Monarchy Class A amps to sound quite good, and Clayton also offers some amps in that range.

If I were to generalize based on my experience, in an attempt to answer your question, I would say well designed Class A amps, when compared to well designed AB amps, offer “sweeter” sounding high frequencies with less tendency towards grain or harshness, seductive midranges that are generally full and rich sounding with excellent harmonics, as well as being more dimensional (although tubes and hybrids do dimensional the best IME), extended decays, full, meaty bass, and a warmth and/or organic nature that adds a level of realism to the listening experience. The better/best Class AB amps can provide these things also, to varying degrees, and tend to sound different, with sharper leading edges (not necessarily in a bad way), more detail at the frequency extremes, better bass dynamics, and perhaps a touch of dryness in the midrange (in comparison). Of course there are exceptions.

If you're not one of the elitist that wont have anything but a class A amp, suggest you try a Magtech amp by Sanders Sound Systems. Wont cost you anything to try it, has a lifetime warranty, reasonable priced, delivers 500 watts per channel at 8 ohms and 900 watts at 4 ohms and and will deliver down to 1/4 ohm and finally runs absolutely cool. Has a 30 day no risk trial. What does one have to lose? Numerous reviews on this amp. See the Sanders Sound Systems website. No, I am not a dealer. I am just a very satisfied customer. IMHO an incredible amp.

everyone is talking about design difference. No one is talking about difference when listening. Can you hear the difference!???

no one ever talks about differences during actual use, because, there are none. As is the case with most 'issues' on this forum. But, since no one has mentioned hemorrhaging ears yet to support their argument, things are improving.