I'll have to look into that design. I was thinking they were some type of single reflex bandpass enclosure. I was thinking about the 18" subs myself. I have the room now for a transmission line enclosure and that may be the route I go. I just need to figure out what else the enclosure could be used for; large table or hide it behind some cabinets.
Go to the Klipsch site and read about the Jubilee:
http://www.klipsch.com/kpt-jubilee-535
Go to the Obsidian site in my signature line and read about the 18" subwoofer drivers. Building TL design boxes are more difficult, costly, and don't give any appreciable advantages over well designed ported boxes. I use to build TL boxes and quit after hearing very good ported boxes. The TL design does give a smooth roll-off, but hearing the extended output that the ported design gives with a very high sound quality driver in it, I will only be building ported boxes in the future. Here is a pic of the (2) 4 ft long boxes that I'm bolting together to make a 8 ft long sub box for my living room which will have the Obsidian 18" woofers in it, tuned right at 20hz.
bolted together
each woofer will have approx 600 watts going to it.

The woofers have great sound quality, built like a tank, and are very affordable. The multiple woofers will all share the load, be more efficient, and coast along down deep as they move their close to 3 inches of excursion.
video of excursion here:
I'll have a 8 ft long Walnut butcher block top, full width black grille cloth panel, with walnut strips on the corners, black plexiglass mirror panels on the ends between the strips of walnut. It will reside right below my 110" diagonal screen.