The main problem with discussions about TL speaker designs is that they are not, and cannot be, true transmission lines*. The handwaving that went on about low speed of sound in the line, and other stuff, was just fantasy. OTOH, the design of a tuned line with carefully placed damping is very much alive. The work described over at
Quarter Wave will tell you more than you probably wanted to know. These speakers seems to yield good results and have a solid mathematical footing. Some of the bass speakers designed using Hornresp are very similar, and there seems to be general agreement between both sets of design regimes and their respective mathematical modelling.
These designs can get you some serious deep powerful bass. The group delay might not be to your taste.
A true transmission line is possible for mid-range drivers. But there you just get a long damped line, and one tweaks the stuffing and measures until the line is dead. No open termination. IMHO such an approach is worth investigating (cf the B&W Nautilus). But you can't use it for real bass. Just silly big.
* As generally understood, a true transmission line is something where the energy put into it never comes back. The finite length of any real line makes this a difficult problem, and they will all behave as either quarter or half wave resonators (depending upon termination.) If you don't put any energy into the line near the quarter wavelength, close the end, and properly damp it, you can get close to a black hole for the rear radiation from a mid-range driver. It still isn't exactly a small enclosure. Short open ended lines for mid-ranges (Statements for instance) are another matter again.