There is already significant number observations for which tests do not exist.
Right now, it is like if someone claims that they see green men, force them participate in a test to see whether they can see blue men and claim they cannot.
If you look at the history of science, models emerge to explain observations, sometimes from scientists in controlled situations, some from regular people in uncontrolled situation. In the latter case, you devise a test to see if the observations can be replicated in a controlled setting, not demand that the regular folk create their own controlled tests before they can report observations. Of course, you need some reasonably credible observations to start with.
But we already have some from the spreading DRC systems. For example, many AVRs give you choice to select from a flat target or some “reference” target curve to do their corrections to. Both smooth the resonances etc but they affect tonal balances. There is clear evidence in all the forums for these equipment that people have preferences for one or the other. Heck, even family members differ in their preference in the same equipment. Other than saying, they are subjective preferences, there is no science based explanation for it. You can either claim that such a subjective preference does not really exist if the test was controlled or have a theory that explains the correlation between what someone prefers and the FR curve that makes it pleasing for that person. So, you are not accounting for experience in real world at all in that case.
So, a valid enquiry is whether they will able to reproduce that preference in a controlled test. For example, play the sounds for each of the curves using the same equipment in a fully controlled test in a random repeating sequence between the two and see if they can pick the one they prefer correctly in a statistically significant way.
If they don’t, then that preference was illusory. If they do, then it needs a theory for why that is the case and it is easy to extend that into why people might prefer one unit over another. Not because of distortion numbers.
As far as I know, none of the existing studies or tests do that exactly. Denying those anecdotal observations because the typical consumer cannot do the controlled test to the level needed to establish the reality is not a defensible position.
You are kind of all over the place...mixing areas where subjective preferences have perfect validity, and where they have none. I'll leave most of that without comment...
The typical consumer you mention not only isn't going to care about any of this, they aren't really who this forum attracts. If someone is so convinced the current measurement and psychoacoustic science doesn't capture what they hear, then the first step is to actually demonstrate that you can hear it, not just claim you can. It doesn't matter how many people make the claims, anecdotal evidence just doesn't convince those of us who have actually gone through the exercise of blind, matched tests. It is hard to have patience with those who just refuse to do it.
If you can hear it, you can measure it. Show otherwise, and claim the Nobel...