As a true sceptic I never believed there was a difference between class a en ab with my Marantz PM 7200, I never heard it also until now.
Just for the fun I had switched the amp to class a mode, having listened only in class ab for as long as I have this amp (only in the beginning I made some comparisons).
Meanwhile, playing some music, I was browsing the internet and when I stopped browsing to put on a new cd and lay down on the couch to really enjoy some music I already had forgotten I switched the amp on class a mode.
I played a cd I know by heart (Richard Buckner's Devotion and Doubt) and then I noticed it sounded a bit peculiar, the bass was very soft and vocals came up and left as if I was under the influence of canabis, very liquid with no texture at all. The soundstage was narrowed down a bit but was held together more tightly, it was all very homogenic.
Stereo image was different: with some vocals the ssss-sounds didn't end at the same spot where they came from, but were smeared out (or smoothened out) to the far right, with no distortion whatsoever, but in class ab it is better controlled and positioned, although not as smooth.
Bass in class a is tighter, but softer and not slow by any means, in ab bigger, fatter and harder.
Class ab is down to earth, with more texture and dryness.
Class a is listening to water n.y.m.p.h.s. (classical) (why is n.y.m.p.h.s. , without the stupid dots, not allowed on ths board????) or listening drugged-down to the Velvet Underground
I prefer ab with most music...class a with Monteverdi's Maria Vespers or Philippe Herreweghe performing Bach for instance