So much to unpack, and I'm wondering if these issues were addressed:I disagree with how you've used the phrase "very few differences" and "quite close results" to describe the variation in treble and bass preferences across gender, listening experience, and country of residence. In this situation, I'll simply provide the direct quote from Olive and Welti for the sake of clarity.
"3. Younger listeners (15-25 years) on average preferred about 1.6 dB more bass and 0.6 dB more treble in their headphones than the older listeners....The oldest listeners (56+ years) preferred the least amount of bass, and more treble than any other younger age group.
4. Listening experience had a significant effect on the preferred levels of bass and treble. Less experienced listeners tended to prefer higher levels of bass and treble than more experienced listeners...
5. Female listeners preferred about 1 dB less bass and 2 dB less treble in their headphones compared to their male counterparts...
6. The German listeners, on average, preferred slightly less (< 1 dB) bass and treble compared to the listeners from Canada, USA, and China."
S. Olive, and T. Welti, "Factors That Influence Listeners’ Preferred Bass and Treble Levels in Headphones," Paper 9382, (2015 October.)
#3. the oldest listeners 56+ years may have required more treble due to hearing loss - did the study investigate the relative hearing loss at higher frequencies? This may explain the preference not so much that they enjoyed more treble but rather required an EQ bump in order hear the treble at all!
#5. my understanding is that female hearing is more attuned and hyper sensitive to the frequencies corresponding to the cries of a baby (aka treble) so I can see why they would prefer less treble - if such is the case, shouldn't the Harman preference curve be gender specific then?