The only reason I even clicked into this thread is because: the OP said he was buying 18's or 21's, and I was here to talk him out of it, to tell him to buy 24's instead...
Then he bought 24's, so my job here is now done... hehe! 😅
The UHT-24 is better than the SI-24 I'm sure... But I'm only interested in the extreme harbottle 24 (the one that doesn't yet exist.) I'm not interested in the UHT, I was out before the 2nd buyer placed their order on round 1. For the reasons I already mentioned months ago (re: linear excursion).
The SI SHS-24 and SAB-24 have silly amounts of one-way at 70% BL. That's the amount of excursion I'm interested in (basically: as much displacement as I can get my hands on), after-all these are ULF drivers... and I'd be using them for mostly(1-20hz.)
All of this I already mentioned (more or less at-least...)
I'm sure it is a great driver, just not for my needs\wants\desires. (Sorry if that doesn't align with anyone else's plans.)
If I didn't have any existing subwoofers I would have probably bought some in round 1, But I do...
I'm already well-beyond cop-calling levels of bass (and have been for years...)
I could very-much use another 12 24's or so, (in HT 1.0 even), but I'm in no hurry for it...