The hiss noise is not volume control dependent on class D’s, which means it does not emanate from the gain stage.
It’s there on some that have too higher set a final roll/off filter, eg flat to 20khz or more. And this lets through the switching noise through to the speakers which is heard as hiss from the tweeters.
The Nad may have filtered it out by setting the final roll/off filter lower (<20khz), and thus subdue the hiss much more.
BTW I had to replace both tweeters on the Wilson 8’s (which weren’t cheap) as both were severely compromised after all that constant punishment over time, needless to say he got rid of the ClassD monoblocks after that, and purchased a good linear amp.
Once the technology is available in the near future to get the switching frequency 3 or more times higher than it is now to say 2 megahertz or more then the output filters can do the right job without getting too close to the audio band and effecting it with early HF roll off and phase shift, as it will be able to be done well away from the audio band and leave it untouched.
Cheers George