You don't have to give up low SPL because you go ported. Porting low (below 20hz) often gives a huge SPL boost at port tune (below 20hz) compared to sealed and provided the tune is low enough (think 17hz, 16hz, 15hz, 14hz etc...) then you have usable real world output down to single digits with the right sub and box.
Bigger boxed have less fall off below port tune so that's a negative, but if you can hide behind a false screen wall and use the height to add some volume you can get a really nice big ported sub with output at 10hz.
Four UXL's are really nice subs. Those ported would be even nicer
Those subs are monsters either way, but they are scary when ported. Check this out:
There's plenty of exension IMO. The point where the ported and sub is equal is about 6.85hz. At 8hz there is more usable output on the ported.
If low extension is your game you just need to lower the tune a bit. Sure you give up some SPL in the meat of the bass band (30hz-60hz) but that is compared to a higher tune like 20hz or 25hz, not a sealed box. Compared to a sealed box the ported hangs with it for SPL no problem, and near port tune has a ton of power handling and output.
You could port at 12hz or 13hz even with those subs and get significant output at 6hz if you wanted it.
The shock factor and impressiveness of 13hz
db is a lot more impressive than 6hz at 90db IMO. By the time a low port tune sub drops in output compared to a sealed I think output on both is dropping off on most subs anyways so a proper port design is a great way to extend out your -3db bass shelf. If your sealed subs are down a bunch of db's anyways it's going to take EQ and a ton more wattage to bring that back up flat- that just puts a heavy load on your amp and driver unnecessarily IMO.
I've been playing with subs and sub ideas for my theater and I am totally stuck. I want Sealed but to get the output I want in my giant room I'd need so many of them and so many watts.
One idea I had was a front loaded or folded horn in my riser, two actually with two UXL 18" drivers and tuned for output to about 25hz to start. I'll lack that ultra low extension but I'll have plenty of output out of just 2 subs. And then later I can add some low tuned ported subs to fill in the bottom. 4 drivers should get me 130db's
and also with usable output at 8hz of 105db. I can't figure out how to get that with Sealed subs.
If I go sealed I'd need to do like 4 subs like you, but then add something like 10 or 12 more cheaper subs like the Infinity 1260's around for a boost. The wow factor of a lot of drivers could be cool, and I could hide them in clever spots. I am just totally confused. I think I need to audition some sealed and ported and horns for some help, I wish I had attended one of the DIY audio guys get together events.