I wouldn't wake up this old thread, except I think there is still more that has been missed.
Irving M. (Bud) Fried was the original importer of IMF speakers (note his initials), and later produced his own under the Fried name. He was a firm believer in transmission lines - I have two models, the IMF Studio III from 1972 and the Fried Studio V from about 1995. They are still in daily use.
I have most of his marketing newsletters from 1961 to around 2000, where he discusses how transmission lines work and why he used them. The attachment "The Virtuous Transmission Line" is most relevant to this thread.
I am attaching the brochure for his System H, which may be one of the first, if not the first, subwoofer / satellite system. For those in a hurry, sections IV and VII are most relevant here. This system must be at least 50 years old by now. (Edit: introduced in 1975.)
A few decades later his enthusiasm for TL's had not wavered.
One assertion is that the back wave from the speaker is attenuated and delayed by placing resistive materials in the line. The goal is to match it to the direct output from the speaker, but coming out from the bottom front vent. He asserts that this creates a coupling between the front and rear speaker energy that produces a planar wave front for the bass, which is much larger than the wavefront produced by the speaker cone alone. He also asserts that this reduces room interaction compared to that produced by a single point source.
"Transmission lines: By their nature, as described in a 1970 article written in "Hi Fi News" by an English associate and myself, lines appear in rooms as planar sources, with much less energy going randomly to the walls (as in "spherical source" loudspeakers). Thus, there is much less room interaction with lines, and purer response no matter where situated in the room."
I also attach some of the marketing materials and newsletters about the Studio V. In the printed blurb at the beginning of this file he discusses the difference between using a transmission line in the bass and what he terms a "line tunnel" for the midrange. There is other stuff in the PDF file that is not relevant to this thread. Sorry, but I had it all bound into a single unit. There is, however, a picture of the transmission line in a cutaway drawing.
Finally, I attach two pages discussing the design of the Studio V in which he further discusses TLs. "The Virtuous Transmission Line" is a blurry copy of an article from "Hi-Fi Heretic", a publication I never heard of!
I hope this sheds some light on the views of an insider and one of the transmission line's biggest proponents.