Well then, thinking outside the box - Peerless by Tymphany STW-350F-188PR01-04 15" High Power Subwoofers, if you can find them, they are amazing!
Peerless by Tymphany STW-350F-188PR01-04 15" High Power SubwooferA New Generation of High Output, High Power SubwoofersThe STW-350F-188PR01-04 subwoofer from Peerless was designed with performance and space in mind. This drivers offer deep and clean low-frequency reproduction with minimal...
15" subs with 7 1/2" voice coils, rated to take 3500 watts RMS, Fs 25.3Hz, 88.7db sensativity, SQL all the way! 4 of these in parallel (1 ohm) instead of 6 12's, your all set! DigiKey direct at $470.02 each. I built two subs for a guy's home theater, ran each with a Crown professional XLi 3500 amp, was simply breathtaking. 2.25" MDF cabs, 4.5" faces, unbelievable sound reporduction on all levels from around 21 to 80Hz. THese babies are real heavy too, you'll likely need 2" thick birch ply with ribs in your application! These units have additional mounting supports on the rear of the drivers too, again, they are heavvvvy!