I've owned excellent versions of all 3... I've owned a dozen or so amplifiers in my time and can tell a difference between every amp that I've owned, But I can't say that it because one was Class A... A/B or D... Great amps are available in every class output. I suspect that if you put 3 excellent examples of a Class A, Class A/B and a Class D in any system, most of us would be pretty happy campers.
I currently own a very highly modified Sumo Nine and an amp with Abletec Mono Modules that I built in a stereo case.
Both are very nice... By the way, anyone out there that would like to venture into Class A inexpensively, the Sumo dead stock is pretty good, but proper parts upgrade really make this amp come to like, incredible value.
Overall, I believe that you could make an easy enough argument for tube type or bipolar vs Mosfet or any other output device. Overall, its the design that matters.
I hope this helps, Tim