Fatigue is an interesting factor as there is no physiological bias involved AFAICT - when you can't listen any more you just know, it's almost physical.

I have noticed different headphones and using a class AB headphone amp vs builtin switching amps can significantly impact how long I can listen for... But that anecdotal correlation is not enough for me to assign blame to switching amps, it might just be crap switching amps... or switching amps with not enough headroom. The problem is class D is always used for cheap low power devices, so most bad experiences are inevitably going to strongly correlate with class-D - whether or not it's attributed to the fundamental property of switching vs just being a bit ... shit.

Blinded or unblinded?

Both. I do not believe that non-blind tests are worthless. When you are dealing with subtle effects, knowing what to listen for actually helps in detecting the difference. However, I tried both blind and non-blind. Did not like both times. I really wanted to like Class-D though, it is not like I was prejudiced.