Yeah, that's a good point. A few others have also reccomended this.

Max linear SPL is that of the graph shown, 30Hz and above in Hornresp shows excursion requirements are minimal, but in the 20-30Hz region, excursion requirements shoot up greatly hence limiting maximum output. A longer horn length will remedy this, at the cost of size :'( I'm going to try and see what I can do though, maybe I can fit in some extra length.

My walls are strong, sort of :P One of the walls has a massive crack in it, many multiple and deep cracks actually which got there when a new door was put in some 40 odd years ago by another owner, seems like the guy didn't quite get the handle of cutting a hole in the wall, haha. My 18" vented box sub has also added to the cracks :P Not to mention the ceiling actually is pulled away from one wall completely, and my sub has extended that down half of another wall too :P We'll see what happens anyway, its held up the last one and a half years I've owned the 18" sub. Another good year of 120db's should bring it down, haha.

Cheers
Adrian