I know what makes a speaker sound to bright: any boost above 5kHz. I've literally got speakers on my desktop that need a -3dB correction for everything above 5kHz to not make them sound bright. Issues in the 9-11kHz range still bother me though, especially if you have loudspeakers without those issues in the same price range.it didn't bother me at all. Also if I use EQ to boost the area above 12Khz on my own system I can hear the difference it makes. So that dog doesn't hunt.
I would suggest everyone tries a bit of EQ just to see what is required to make a speaker unpleasantly bright. IME you need to boost between 1Khz and 5Khz, above that you just get 'air and space' and, although that boost can sound un-natural, it will ofc depend on the speaker/room balance that you start with.
Treble lift way up in the 10Khz plus region just does not sound 'bright'. Yet we see this 'bright' assertion on a lot of the speaker reviews even when the lift is only above 15Khz.
That's the limit of FM radio. Do youngsters think FM sounds 'dull'?
Real world experience is invaluable for comparing speaker measurements to what will be perceived.
The fact that you can hear 12kHz when you boost it doesn't really prove a point, hearing loss doesn't mean you can't hear the frequency at all. It is a combination of the frequency and the spl level you can hear it at. A younger person will hear that 12kHz while its 10-20dB quieter than you can hear it at.