@stringreen re the difference between brightness and clarity you are spot on. Your definition of clarity lines up exactly with what I got adding a super-tweeter to my system -- definitely not 'bright' but much more insight into the recorded space and the different elements of the performance -- more details on my experience here
https://forum.audiogon.com/discussions/supertweeters-yes-please
https://forum.audiogon.com/discussions/supertweeters-yes-please