they did many things, it's too bad that the papers were only available for free a short period of time after each was published. but you can probably find Sean Olive talking about some of it in videos. they tested age and found no significant difference, they went to several continents to see if there could be cultural or genetic differences and apparently didn't find any(with the caveat that they didn't go test isolated population deep in the amazonian forest or that kind of stuff). I had many critics(and praises) about their job when they presented the first papers, because I thought that was it and it was clearly incomplete. but as the years went by and they kept testing new stuff depending on the results of previous tests and questions they came up with(you know, real science). so I can say that it's probably the most extensive and serious study about subjective preference of headphone signature. I doubt any internal study by any big firm went as deep as they did with as much rigor. I don't remember specifically when the listeners could pick the songs or if the choice was limited in genre, so I don't want to tell you something wrong. but I would assume that they have answered that type of problem at least a decade or 2 ago with their extensive research on listening tests with speakers. my guess would be that there is a matter of music genre involved but that it falls under the vast mystery of audio's circle of confusion, where at some point people master a certain type of music because they expect it to be played mostly on certain types of systems, and soon enough people start thinking that they need a system with a specific signature to best enjoy that genre, close the loop and enjoy ^_^.