Transmission line Subwoofers?

There are a bunch of DIY subwoofer designs on the web using TL enclosures, including one that (I believe) Teres Audio will build for you. If memory serves, Wisdom Audio (used to?) sells a finished product, as well. Not a lot of choices out there AFAIK.

My local high end dealer has TBI subs set up for demo. I find them a little quirky sounding. I think some folks will love the "bloom" (lousy descriptor, but best I can come up with) that they provide in the bass, but not entirely convincing to me. Certainly not bad, and lots of power from a small enclosure, but definitely a try before you buy product IMO.