Do what I did. Build your own subs.
8 15" subwoofers with ~4.8K Watts bridged in a series parallel combination.
2 12" midbass/midrange drivers with EV compression drivers powered with ~2.4KW
For me, I prefer sealed subwoofer boxes with a Q of about 0.9. Vented boxes are more efficient and handle more power but if you want realllly deep base. 15Hz... You want a sealed box. 99% of vented boxes unload the driver at high power below 30Hz. They do nothing but destroy themselves from over excursion if you try to turn them up.
This is why I like the simple sealed box. The driver never unloads and this means you can still produce very deep, infrasonic sounds. Yes, you can equalize it flat for response as low as you want to go and the cone always remains in control of itself with proper supervision.
With a vented box, it doesn't matter how much supervision you give them. They just won't reproduce much large signal sound below their resonance frequency.
I like sealed boxes because there are occasional movies or music clips with really deep bass in the 15Hz - 30Hz range. When you have high power amplifiers to back up the sealed boxes, you can crank the "F" out of it at high levels.... And that is when things gets fun.
Its not necessarily (audibly) loud at 15Hz, but when you dump 4.8KW of infrasonic energy into the outdoor theater air, its absolute bliss. Even when you crank the "F" out of infrasonics, its not audibly loud, but you absolutely feel its warming glow.
Have fun. Do it.
PS - High power levels @ 15-20 HZ won't bring the cops to your house like 100Hz will.