Why do I like Class AB better than Class A?

marketing. I would bet you a lunch that the Plinius is biased class A up to a particular bias point and even within class A/B and when you flick the switch the bias is reset to a higher bias point.

enjoy

Got it. Still once you turn a transistor on, it is on. It can be turned on at a really low bias point, but, it is still on. Then you can really turn it on at a much higher bias point where you are cooking at full rated output without an input signal.

This is marketing and semantics. But on is on. Some devices are biased such that they aren't turned on until the AC signal is passed through it. Others are biased such that they are turned on with DC bias with or without an AC signal passing through it. Again, many of the amps I have tested and repaired state they are class A on the specifications and are rated at 50, 100 150,etc. wpc. This does not mean that they are biased at the rated output capacity. They are biased on at 2 watts or 5 watts of 20 watts etc.

also, thanks for the clarification. I didn't know this amp had a switch that altered the internal circuit bias. However, to be clear, all it did was change the bias point from a lower to higher output driver bias. Which is a good circuit design idea as long as you have the power supply capacity and the proper heatsinking for the output drivers. Full output class A is wonderful for those cold days and nights when I want an extra space heater. Tube amps are also good for this.

enjoy

I don't think Plinius designed the SA10x and SA Reference/250 to be Class AB amps but they wanted the amps to be left on all the time, so providing that AB switch provided owners a way to leave it on 24/7 without busting the electricity bill.

FWIW, when comparing Threshold model amps that had similar chassis, though with differing bias, etc., that were sold as different models with either Class A or Class A/AB (Class AB) output; other than running hotter and with lower total power output, the Class A amps were smoother and more laid back, while the Class A/AB (Class AB) amps ran cooler, had more total power output, were more dynamic and more upfront, though not as suave.
Depending on the speakers, the room, preferred live musical event seating, sonic priorities, and of course budget, I could imagine different people having different preferences.
FWIW, I'd prefer the best of both worlds, cool running, dynamic, neither laid back nor forward, suave sounding, Class W hatever. So far it's typically been high powered Class A amps that have come closet to most of those ideals (except of course for the heat, which I could most easily make an exception for), unfortunately the costs of such amps have just as typically escaped me. High biased Class AB amps have so far become the prudent choice, at least for me. YMMV.
Class D might change all that, we shall see.