I don’t think Martin Logan makes transmission line boxes and not sure that Zu or Audio Physic use transmission lines either, although PMC is famous for that.
Lots of ways to skin the bass cat. For instance, Daedalus loudspeakers uses aperiodic loading and this technique, which is basically resistive venting, works great with extended bass for a relatively compact enclosure, which maintaining very high sensitivity, which most TL’s do not have.
My own Altec 604 based speakers are vented, but only to let the driver do what its designed to do, which is breathe freely and not designed to extend bass response.
In my post about Dr. Suess and HiFi, I mentioned the work on loudspeaker enclosures by British engineer Leslie Bucknell while at the company my father worked for, Stromberg Carlson.
Bucknell’s approach to loudspeaker design was to create a speaker enclosure that would eliminate distortions that occur because of cancellations and additions due to standing waves internal to the cabinet. By carefully controlling the way that the sound waves travel through the speaker enclosure—routing them through a complex maze of tuned baffles—he felt that his Labyrinth design was the cat’s meow.
During this same time period, another British-born engineer, Arthur Bailey, was taking a slightly different tack to speaker design called the Transmission Line.
The transmission line uses a long narrow folded duct behind the woofer. The woofer’s output travels through this unimpeded maze until it exits out of the port. This technique differs from the traditional port (basically, a tuned hole in the speaker enclosure) because it is carefully calculated to arrive in phase and add to the low frequency performance of the speaker. In addition, as sound waves travel through the transmission line, they are gradually damped and absorbed, which helps to eliminate resonances and other distortions.
Of the two approaches, it is the work of Arthur Bailey that lives on today in speaker designs like those of PMC, Martin Logan, Audio Physics, and Zu Audio, who all (best I can tell) still use the transmission line approach to making better bass.