Bag End Infrasub 18 (and woofer..)  

The Bag-End subs operate on a principle known as ELF- extended low frequency. This puts a large woofer in a small sealed box where the mechanical resonance of the woofer/box system is at the high end of the system bandwidth. They're sealed, so you need several 18"s to get any sort of output below 20 hz.

There's a great article on the subject here: http://sound.westhost.com/project48.htm

In summary, that woofer is likely to have good excursion and sensitivity, but lousy T/S parameters for normal use. If you can follow the article I linked to, and you're able to perform the measurements involved in fine-tuning the circuit, I say go for it.

I am building something similar to a BAG END Infra 12.http://www.avsforum.com/avs-vb/showthread.php?t=705711 I have read lots of reviews of the BAG ENDS and all give them hi praise for sound quality and extension but they have limitations as far as maximum SPL.For it to work properly you will also need to build the control unit described by Rod Elliot.