How about a cheap triple 12" Infinity box for near field low frequency brutality ?
4 feet wide.
4 feet deep.
About 18 cubes total. Driver on each side. port 45" wide (48" minus sides and brace) and about 66" long (this is about the max) gets you about 12hz tune with 1.75" height. This is doable. The chuffing will be minimal will all but max output at very low frequency. A 8hz sine wave at drivers max wattage would get you 23.5 m/s air velocity - lower wattage or higher frequency you are safe. Just put an 8hz subsonic on it. You can pound these things in that case without worries.
Drivers cost only $185 too. Good for 110db @10hz from only 1000 watts. This is just one box. Two would be formidable. That is 6 drivers, basically a proper bass solution for most sane people in and unto itself.
120db+ higher up in frequency too if you wanted it.
actually it makes sense to do 4 of them... two each 48" side, so you can run 2ohm loads easy of an amp - and run 4 of them off a single cheap amp like a small inuke or something at 2ohm channel.
Put the port facing you, or turn it. I guess it does't really matter. (4) 12" ported at 12hz near field will be felt if that is the effect you are looking for. You are not up to $250 per box in drivers cost. Doubt you can beat the result with a single SI18" - or make the tune work.
What you would want to do with a set up like this is boost the port tune for that low frequency brutality - but also do a hard cut (or multiple) just below port tune to keep excursion in check. You would also want to cut a little above port tune where the drive would be hitting xmax to keep that in check, something like 20hz or 30hz probably. This will make the response very heavy at 12hz (probably 105db-110db) but it will drop off a bit higher (where your other subs do the work anyways)
It could work with the right EQ touch. Keep excursion in check with a high pass subsonic filter below tune, and cut above port tune where the driver excursion might be problematic at maximum power. This would allow you dump full power on them and get significant 12hz output. If you could make the box a bit taller you could get a lower tune and extend the port to get that. But 12hz is pretty decent. The first port resonance would be above 100hz too.
I am just confused- so you want only 10-20hz output for bottom fill and feeling or will you run these full range with your current set up for extra bump in SPL ?
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."