I wish there was a research on how potentially hearing varies in different people. And how we are sensitive to different frequencies. I for one can never understand how someone would enjoy listening to music on something like the Dusk or would call the tuning on Clairvoyance perfect. It must come down more to just taste and music preference.
Interesting topic for sure. Unless there's financial incentive, though, it's really hard to get research studies funded.
At least for the majority of people, I think it's as simple as "good sound is good sound". If you look at the limited research on tonal preferences by Harman, you can see that they found no significant correlation between ethnicity, age, and what was most preferable tonally. Again, this is limited research: a heavily skewed male to female ratio, much more trained listeners from the USA which is why the USA consistently scored the headphones lower, and a small sample size particularly when it came to older listeners.
These are also averages, so they only reflect the majority. They do not address the minority and why those people might have preferred one headphone over another when the majority did not. That sounds like what you're interested in; something like older individuals preferring more treble also comes to mind. But I think most people, as a whole, can agree on good sound when they hear it. Likewise, regardless of the genre I was listening to, I know I'd think some IEMs are bad and others aren't.
What might be even more interesting is
technicalities and how people perceive them. Some people don't seem to hear speed, resolution, or dynamics. Why is that? Some people seem to prefer stuff that, objectively, measures poorly. To go back to the Campfire Audio stuff, I think their dynamic drivers used for bass are bloated, slow, and textureless, while plenty of others don't! Could these people simply be keying in on non-linear variables to the bass that they find enjoyable, perhaps akin to added distortion with tubes? And if this is the case, then what is the threshold of distortion for which that becomes my perception of the bass? If I handed them, say, an IER-Z1R for comparison, which I think has higher quality bass, would they recognize that it's cleaner and more impactful? That's just some food for thought. Ultimately, there's no right or wrong when it comes to what people find enjoyable even if it doesn't measure nicely. I also don't think we'll find answers to this stuff anytime soon, even it's fun to think about it sometimes.
U6t seems quite attractive. 
