ihifi,
This roll off at the top is pretty much standard with high powered amps no matter the class of the design. In a digital amp it is to keep the clock out of the output and in an A/B amp it is to stop ringing. Below I have posted a graph we took of a competitor's high-end class A/B amp and you can see it exhibits exactly the same roll off. The limit of human hearing is around 14 – 16 kHz so you are out of the audible range before you get beyond 0.2 dB of drop. Even if you could hear to 20 kHz then 0.6 dB of a drop would also be inaudible. Hope this answers this one for you.
