Your hearing is fine. Darko admits that if there are differences, they are very minor. Any sighted testing is going to almost always produce a difference, as it does with everything. Only blind testing is valid to accurately determine if there is a difference in sound.
How easily can you distinguish between different DACs?
When I read reviews or watch them on YouTube the reviewers talk about the vast differences between various DACs. I haven't compared too many, but found the differences pretty subtle, at best.
Which got me into thinking: Is my hearing ability really that bad?
Do you notice the differences as easily as folks make out?