There is another way you could test this with your own gear. If you own some other open headphones with appreciably higher distortion, EQ both the Verum 2 and high distortion headphone to the same frequency response. Level match them. And then listen to them to see if there is any difference in the perceived clarity, imaging, and spaciousness.
Ideally, both headphones should also have similar left-right symmetry, since this can also effect the imaging precision. And the testing should be blind. But even if they are not, I think you should still be able to get some sense of the differences in clarity, imaging, and spaciness when switching between them… if there is indeed any.
Imo, the stereo imaging on a headphone with higher distortion will be less clear and precise, and more diffused. And it will also sound more narrow and less spacious. I suspect this is because some of the more subtle spatial cues in the recording are being somewhat undermined by the distortion.
If you can, try it with more than just two headphones. Because the distortion may be different on them. If any of the headphones sound like the imaging is off-center, you can do some basic symmetry correction with left-right balance or gain controls. The headphones should also have sufficiently high impedance and damping factors to mitigate any source-based distortion from the amp.
I am with you, btw, in thinking that there is probably more to things like distortion than this rather small study suggests. I think most listeners aren’t really all that aware or sensitized to how it might effect their content though. And what kind of things they should be listening for to pick up on it. So it also does not surprise me that there were only a couple listeners in this study that seem to be able to hear it.
We are talking about subtle effects though that might also be difficult to pick up in binaural recordings played through another pair of headphones with its own unique sound profile.
I think its an interesting study btw. But I think the subject of distortion needs more research than what is contained here. And think some of the conclusions could be premature or incomplete.