How many audiophiles or sane people who love music actually know, in black & white, how well they hear through the entire audio band? I would bet that most have done a few frequency sweeps and proclaimed their hearing to be "Excellent!", yet they do nothing to fix the acoustics in their rooms.
I used to think it would be a good idea to have customers go in for a hearing test, so their speakers could be matched to their hearing and then, I decided it would be a disaster if anyone else listens to their system in the event that it's tailored to the hearing of someone who has severe deficits.
One thing about speaker audiographs that makes a simple "The response is +/-4dB across the band is that if the +4 of one speaker corresponds to the -4dB of another and the demonstration involves switching between them, something is gonna sound like crap if the widest variations are where humans are most sensitive.
I don't think that it's particularly relevant how well someone can hear with regard to frequency (within reason). If someone has a dip at x frequency, that person will still hear a piano note a certain way live, and that person should expect to hear that same note at the same level (and it's harmonics, which tells us it's a piano) when reproduced, even if that level is not the same as "normal" hearing. If the response is elevated there, he will still hear it as elevated ... few people, especially people in this hobby, have zero hearing at frequencies below, say 15 KHz, they just have a dip somewhere. They can still hear something, and it's relative to what they expect to hear.
It's *individual* responses when dealing with one person's passion or poison.
It's not the same thing as when we do measurements that represent how well this or that component comes to flat frequency response, insofar as that's a metric that is relevant to "everyone", or more correctly the set of people that are HiFi enthusiasts, or accuracy versus live music or the studio mix. In some cases a flat frequency response is an ideal that is rarely if ever met (loudspeakers). Yet we still find loudspeakers whose sound we enjoy.
We also see this in the current literature, where it's found that even people with "normal" hearing are relatively insensitive to frequency response aberrations; tolerating deviations at levels that would not be tolerated well were they other metrics (high order harmonic distortions, noise level, etc).
One final clarification: I'm not suggesting frequency response is irrelevant, or that speakers with smooth, flat response are not better than those with more ragged measured curves. It's just that a "good" speaker sounds good to people with excellent hearing, and to people with some hearing damage. What you can or can't hear will be the same, live or at home, so for individuals, their particular individual frequency perception won't, or shouldn't, affect their ability to hear differences between components.