Hello tmac1700! Happy New Year! Many people (some of whom claim to be amp designers) do not understand amplifier "classes." It all concerns how much of the input wave comes out of the output device (tube or tranistor). If the output is a true replica of the wave, only much stronger (larger), the the amp is Class A. But it is working just as hard with no input signal at all as it does at full musical output. If only (exactly) one half of the input signal is being amplified (the positive or negative portion), it's Class B and at least two output devices are used, one for each half of the input signal. That's why they call it "push pull." So each tube is working only half as hard, but you're getting four times more output power! (It's math.) The old juke boxes had amps like that, along with amps used in the early days of "talking" movie theaters. Class C amps are used in radio transmitters for reasons beyond the scope of your question. Class D,G, and H are different and are aimed at being more efficient in their use of power.
Let's talk tubes, because it's easier. Tubes get used up because a coating on their cathodes is actualy consumed over their lifetime. How "hard" you "bias" it (cause current to flow through it even when it is NOT producing music) affects its life. As you might guess, Class A/B means the tube is not conducting enough current to fully reproduce the input signal when you want the music to play really loud, but - when the music is playing softly (most folks listen at an average of 0.5 to 5 watts) the tube is working hard enough to fully reproduce the input signal. A tube running Class A will have a life about 1/10th to 1/5th (or less!) of the life of a tube running class A/B. I have a Marantz 8B power amp running A/B with its original tubes (way over 30 years old) and it sounds fine. But if you measure its tubes, you'll find they are pretty tired! I'm sure it doesn't produce its full rated output anymore, but I'm easy on it. If you run an amp in the Class A, it consumes a lot of power and runs hot because it's running "full bore" all the time. Incidentally, all amps will produce way more than their rated power but will distort badly and sound terrible, which is why an output rating always specifies a distortion figure - 40 watts at 0.1% harmonic distortion, for instance. It can probably produce 100 watts at 50% distortion! It will be LOUD but sound terrible! (Many electric guitar players don't care!) But if the sound is cymbal crash you may not notice. 160 watts is only 6 db louder than 40 watts. So the guy who says, "I upgraded from a 40 watt amp to a 65 watt amp" won't hear a significant difference in loudness. If you're going to up your wattage, increase by 10x or more. 100 watts is only 10 db louder than 10 watts and 20 db louder than 1 watt. Now you know why people love their 8 watt, Class A, 300B tube amps! An occasional distorted, 10 watt peak isn't much to worry about if you normally listen at one half of a watt! Happy Listening.