Fatmarley
"It appears my intelligence circuits have melted"
I just searched Google Images for SHL5+ frequency response and Stereophile has measured them:
https://www.stereophile.com/content/harbeth-super-hl5plus-loudspeaker-measurements
They look pretty wild up top, then again I've seen other Stereophile measured speakers show similar characteristics so I'm not sure how much trust I'd place in their measurements!
That breakup beyond 20khz is quite normal for a metal tweeter.
What I read over at diyaudio was that when the breakup is excited, it causes problems lower down in the frequency range. I have some hard dome tweeters here that can sometimes sound like there's too much going on when the music gets busy. If I drop the level they sound dull. I spent a long time trying to voice them but I now realise it's just the way they are - No more hard dome tweeters for me.