Anyone know about THT subwoofers?

I've heard that these massive horn subs that supposedly give extremly high output, low distortion, high efficiency, and they go deep, with the trade offs of being massive and harder to build than standard subs.

http://www.billfitzmaurice.com/THT.html

They can be built for under $400 and it's said that they outperform $2000 subs, and even theater systems.

Anyone have experience with these? Any advice?

I've never heard one, but talk on other forums say its pretty good. This is one version of it: http://www.avsforum.com/avs-vb/showthread.php?t=1194093

He also linked his build process: http://billfitzmaurice.info/forum/viewtopic.php?f=5&t=8589