Keep in mind that nearly all audio amplification is done in Class A: Class AB or Class D stages are only used in the final stage of amplification to drive inefficient loudspeakers. (Speakers with a 1% conversion efficiency are actually "efficient" as most loudspeakers go. 0.3% to 0.5% are more typical figures.)
Let’s break down a typical amplification chain:
* DAC conversion element, whether R2R, FPGA, or single-chip conversion -> lowpass filter (class A) -> buffer stage (class A)
* Linestage: single gain or buffer stage (class A)
* Power amplifier, which are typically 3-stage, whether transistor or tube: input stage (class A) -> driver (class A) -> output stage (class AB) -> loudspeaker
Note the only element of this signal chain that is Class AB is the final stage that directly drives the loudspeaker. Everything else, whether tube or transistor, is Class A. In a studio environment, with far more complex electronics, the same applies: Class AB is only used for final drive for loudspeakers, and not anywhere else, no matter how complex.
The reason is simple: everything except for speaker drive is very low power, and operating at low currents (microamps to milliamps). So it doesn’t get very hot unless you have a big studio console, or a rack full of effects boxes. So efficiency doesn’t matter. When efficiency is unimportant, Class A is the invariable choice in professional and consumer equipment.
But ... if you need a 100 to 200 watts to wake up a loudspeaker, efficiency does matter. A lot. My home theater system has a 2010 vintage Marantz MM8003 power amplifier, with a THX Ultra certification for 140 watts/channel. That doesn’t mean it sounds good or anything, merely that it cranks out 140 watts from 20Hz to 20kHz at specified distortion ... that it works as claimed. So far so good.
Suppose this was a Class A stereo amplifier, instead of Class AB. As mentioned in the Wikipedia link posted above, 25% is the best efficiency we can hope for in a Class A transistor amps. So our 280 watt stereo amp must dissipate 280 * 4 = 1,120 watts, all the time, from the heat sink. Uh oh. I can tell you that is a gigantic heat sink, about the size of a medium-size HDTV. Also a pretty good room heater, that’s plenty warm any time the amp is on. Hope you have good air conditioning, because you’ll need it. Or ... the amp can be normal consumer size, or rack-mountable, but those fans are going to be moving a lot of air. Think gamer PCs, with top-of-the-line video cards, noise levels.
Many manufacturers claim sliding-bias Class AB is Class A. It isn’t. That’s marketing talking. True Class A is inherently inefficient and gets hot, whether tube or transistor. If genuine Class A is desired, versus inflated marketing claims, get used to 20 to 30 watts/channel with an amp that still gets plenty warm.
The bad actor is the speaker, not the power amplifier. A miserable 1% conversion efficiency comes out as 92 dB/meter/watt, or in other words, 100 watts of electrical energy is converted into one watt of acoustical energy (which is very loud). It gets worse: the other 99 watts do nothing but heat the voice coil.
Which isn’t good, because the voice coil is very small, with an area of only a few square inches, and is deep inside the magnet structure. For heat to escape, the voice coil must then heat the magnet, and the magnet, in turn, heats the air in the enclosure. Of all those expensive watts that come from our treasured amplifier, 99% are wasted heating the voice coil. And most speakers on the market are even less efficient, typically 0.3% to 0.5%.
In loudspeakers, there is a direct tradeoff between size and efficiency. Bookshelf speakers will always be inefficient. Sometimes called Hoffman’s Iron Law, it is also expressed as an equation in the Theile/Small formulas that define bass performance. No way around it.
So you have a big amp, or big speakers, or Class AB or Class D. You choose.
