Any system can be described as an input times some sort of transfer function (non linearity). Audio systems except for processors are to reproduce the input signal which is the recording. Hence any system that has nonlinearities under certain threshold will sound absolutely the same(if volume matched).
So then that explains why all the class d amps I've heard sound similar to each other. But they all sound similar. While the ab amps sound similar to each other too. Is that the designers deliberately making the different groups sound's for product differentiation then.
It seems to me that the class a amps have the the designers, in recent years, designing them to sound more like the ab amps, in other words, less swarm and syrupy.