I have been very pleased with the Outlaw LFM1 Plus sub. It's design includes two down ports of which one can be plugged for the 18 Hz region. I'd keep this one under consideration.

FireGuy
M22V3/M3ti/M2V3/Omega Super 5/Aperion 5C/OutlawLFM1+Sub/Denon AVR1906/YamDVDC750/AS AMP110