Sorry, hope my answer didn't come across as supercilious.
I don't know very much about transmission line speakers, but my understanding is that the transmission line functions as a "folded horn" inside the cabinet, allowing a smaller driver to produce a lower fundamental bass response than other non t-line designs, e.g. acoustic suspension.
In particular, "transmission line" is the name of a specific audio speaker enclosure topology, in which sound from the back of the bass speaker chassis passes along a
long (generally convoluted) path within the speaker enclosure. The energy is absorbed on this path, or emerges from the open end in phase with the sound radiated from the front of the driver, enhancing the output level at low frequencies. Note the key word in this description is a "long" path.
http://upload.wikimedia.org/wikipedi...ssion-line.png
Note that in this diagram, the bass driver is mounted up high in the cabinet to help provide additional length to the t-line, just as it is in the 24.
The reason this toplogy has not been utilized to the extent that acoustic suspension or bass reflex designs have is that it requires a more complex internal cabinet structure and therefore is more expensive to manufacture. For mass-produced low or mid-fi speakers, it simply cheaper to put a larger driver in a simple enclosed or ported box.
So, while my answer may have at first seemed supercilious, it is in fact the length of the t-line that allows a small 6.5 bass driver to produce bass response down to 28 Hz.
. The design, shape, folding and port relationships are also critical adjustment parameters for the critical functional response, namely the bass response.
Hey, Jerome, it just occurred to me that one could apply Design of Experiments (DOE) here, too! (see my thread on DOE for sub integration in the general audio discussion forum).