As you say, the evidence we have suggests.... but it doesn't suggest that everyone will share the same preference in controlled testing correct? Only a small subset of people?
A
large majority of people show similar preferences in controlled testing, and
it is not a "BBC dip" that is preferred. If even a small minority prefer the BBC dip
consistently in controlled testing for domestic listening - and you can demonstrate that: only then is it really relevant to start discussing why (theories/hypotheses have been put forward for it, of course). At the moment we don't have evidence for that proposition. As I said, you have to show people with consistent preference for the dip before you can start to consider why in a meaningful way.
You really should read Toole's book before continuing on this course. I think you need to see the full proposition, rather than someone firing off a few graphs or numbers at you.
For my part, if I come across someone who prefers a speaker with the dip in a sighted preference, and is enjoying listening to music. I don't really feel the need to fight that. It's not a crime or anything, and while I might not go there (I did own a speaker with a slight dip in the past) I'd still listen happily to that kind of system for the most part. I would recommend that newcomers to hifi listen first to speakers that do match the majority preference, and only move to those that don't once they find that several of the speakers that do miss the mark for them in a similar way, which would point towards a different preference, and probably the same for any listener disgruntled with their current system. I get that people are different from each other, but in audio we are not
so different as you might think.
Remember that the BBC speakers were developed for broadcast monitoring, a different purpose to recreational listening in the home.