Given your needs/wants, your large open space, and the fact that the OS LFU wasn't nearly enough for you, I'd say you might have to step it up to more horns or multiple large, ported boxes. I don't think 4 of even the best 18's in sealed or small ported enclosures would be enough for you. So, I'd either look at picking up some used horns, or buy/build some LLT's with some 21's. If DIY doesn't work for you and you don't want massive boxes, maybe you could check out the WW Speaker Cabinet/Seaton collaboration. Search for X21.
If you go with the HST's, you might be limited in the higher frequencies. You could always supplement with some PA460's for midbass for cheap, but then you're talking more boxes. You could also spend more on 18's that would have more consistent output through all frequencies, but at that point I'd rather go with larger drivers. Tough call!
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