Agree with Sean on virtually all of his points. I also never heard the Steve@HiFiFarm definition of Transmission Line.
In response to a few of the others' questions above, the original true Transmission Line was a cabinet design to "get rid" of the back-wave, and have a better loading of the woofer at the point that it would normally have an impedance spike. The original Transmission Lines did not have any output form the port, at all, and were designed specifically so they would not. Tapered transmission lines that taper down toward the port have better low end extension and linearity than straight pipes, or tapering wider toward the port. Sounds that do exit the port may be in phase, but are not "in-phase at all frequencies" with the direct radiations of the cone. Cancellation and addition, known as "comb-filtering" can occur. Since most modern "transmission line" speakers are quite a departure from the original idea, they should more aptly be called "modified transmission lines" or maybe some "catchy" name for marketing purposes.
I use a TQWT Voigt Pipe enclosure which is strange combination of bass-reflex, transmission-line, and back-horn. It makes use of some of the characteristics of all these designs in a "blend" that looks to be horrible on paper, but sounds great in the listening room.
Transmission line design is still a time consuming job that needs alot of work to get it right, and doesn't really lend itself to mass-market applications.
