You should build a Ghorn and put an SI-18 in it.
The best-case scenario is that it works wonders and you have more output than a UXL-18 without having to buy them.
Worst-case scenario is that you blow an SI-18 up and are out $159.
and in that case, buy the UXL-18 and put it in the GHorn.
Above 16hz the GHorn outputs up to 128db with room-gain, and is -3db @ 13hz.
Watch lukes dual GHorn video:
I couldn't even imagine 4 GHorn's... even based on a weak driver like the SI-18.
When you are generating 150db @ 20hz you'll forget all about 10hz!
If the GHorn is too big you could try a LHorn,
or if you are feeling really adventurous: prove out my HzHorn for me.