The ear can hear what is lower distortion. Those that listen to parts and execution absolutely know this. Most manufactures know this. This is why you see more expensive parts being used.....it is not just to "sell" you something....it is mostly because they have spent hours and hours trying to find the lowest distortion sounding parts to make the sound the best.
It has been widely proven in scientific studies that certain types of "euphonic" distortion are actually preferable by many over a lack of such distortion. The fact that many people prefer certain tube amps with much higher distortion levels than other available amps proves your thesis that "they have spent hours and hours trying to find the lowest distortion sounding parts to make the sound the best" is wrong.
By your logic, the amp with the lowest distortion sounds the best. Halcro tried that. It didn't work. It is much more complicated than "the amp with the lowest distortion" sounds "best".
Once distortion levels are below audibility, it doesn't much matter in any case.
This amp has rising distortion with load impedance, much like other class d amps which do not have the output filter inside the feedback loop. It makes their frequency response load dependent. This is yesterday's class d amp. Smart designers like Bruno Putzeys have the output filter within the feedback loop to create a flat response with load impedance and reduce distortion below audible levels.