Sorry for the case of mistaken identity Brad (I think I've done that to you here before too).
Here's the best explaination I've seen for transmission lines:
http://geocities.com/rbrines1/Bob Brines hand builds floormounted speakers from the deck of his Memphis apartment and out of the trunk of his car, so he does bunches of design on paper before "making sawdust".
I've owned a pair of transmission line bass units that I built from a kit which used 8 inch drivers. They were magnificant instruments, but alas were too much for any size of room I could afford. So yes, they make great subs. In fact the cabinet design of the speakers I'm using now look a lot like the FT1600. Bob explains that they even out the impedence hump of typical designs and they extend bass. But even he would admit that an infinite baffle is the best design for deep bass.
Here's a forum dedicated to infinite baffles:
http://f20.parsimony.net/forum36475/Until you've heard it, you probably can't believe how over rated the top and bottom octaves are or how coherent (right) a single driver speaker can sound. No crossover to drain power or add phasing errors. And single driver speakers are the orginal "active" speaker with all the advantages of the amp "seeing" the imposed load better. Single drivers provide the theoretically perfect point source for ideal imaging/soundstaging. Its a throwback to days past where recordings didn't go much past 80 - 8,000 Hz and amps were limited to a few watts (so you didn't waste power on deep bass). Just another very interesting and fun aspect of this hobby. And it's counter to most of the marketing hype out that says you spend tons of money on 3 or more drivers with crossovers and bigger amps.
Check out Omega Speakers and Solar Audio here at audiocircle (more single driver speakers). Omega, like the new Decware speaker, uses Fostex drivers that are reasonably priced paper cones. Solar, like Konus Audio or Carolina Audio, uses a remarkable 4 inch diameter E. J. Jordan 92S aluminum foil driver in transmission line cabinets.
Lowther's were the original full/extended range drivers. They are faster than ribbons or electrostatics and have been around for 70+ years. Supervox, AEC, and PHY are other high quality European brands of extended range drivers. All these are quite expensive, but are very efficient (95 - 105 dB/w/m).
The biggest challenge with full/extended range drivers typically is getting sufficient bass output, hence the use of open/infinite baffles, transmission lines, and front/rear horn cabinet designs.
