We can say with some confidence 10% distortion is audible, but is 0.1%?
And, compared to what? Can we hear the difference between 0.1% THD and 0.0001% THD?
There comes a point in measurements where one must decide what matters and what does not. It probably doesn’t matter, for example, that one wheel of your car might be 1/10th of an inch different in diameter than another. On the other hand, 1/10th of an inch parts variability in a Swiss watch might be the difference between functioning and non-functioning.
From my perspective, once we hit THD and IM levels below 0.1% the differences become more academic than audible.
And yet we hear major differences between differing topologies producing dramatically different levels of distortion.
What that suggests is simple. Beyond a certain point, the level of distortion becomes meaningless.
How those differing levels are achieved is where we find the bulk of sonic differences.