When I had a Pass XA30.5, whenever it would shift to class A/B, the sound became very slightly hard or mechanical. It was barely noticeable, but over sustained periods I could tell that something just didn't sound right. It's difficult to pinpoint, but I'd describe it as a slight lessening of naturalness and flow. Of course, this is just my experience with one amp and there are likely other things at play (like distortion, both with the amp and speakers). I think it would be very difficult to prove either way whether class A or class A/B is better or worse than the other, to our ears. This is very subjective with a lot of variables.