I've modeled them and so I know what the models say.
In the real world I've heard 4 SI18s powered by my SpeakerPower SP2-8000 amp and my pair of LMS-5400s powered by the same amp and they were equivalent. Actually, the person with the 4SI18s had them powered by a pair of EP4000s and I was underwhelmed after hearing my LMS-5400s the night before. That is when we hooked up my amp and heard what I expected, which is 2 SI18s is about equivalent to a single LMS-5400.
Then at Beasts GTG we put the LMS-5400s against the HS24 and the HS24 dug deeper and played louder. Sound quality was about the same but output definitely favored the HS24.
The best "system" I've heard was Beasts system with Danley SH50s and his 8 SI18s in DO cabinets. Phenomenal. So the SI18s are a tremendous value if you can fit quite a few but the HS24 is at an entirely different level. Since I would never recommend a single sub, I'd recommend multiples, I'd recommend a pair of HS24 as being plenty for most rooms and people. If you can't do a pair of them then do a pair or more of another sub like the UXL-18 or quite a few SI18s. Then focus on speakers, projector, screen, treatments, etc. as your bass would be handled.