First speakers I built were the Roger Sanders transmission line design published in Speaker Builder magazine back in 1980. Being dirt poor they were built to fit the 10" woofers from my old JBL L26 Jubal speakers.
These transmission lines were about 14" wide by 20" deep and about 48" tall. This was way before all the speaker building software we take for granted today. A lot of this stuff was worked out pencil and paper. However he did it the same drivers that put out lumpy bass that trailed off below about 60Hz somehow were a lot more flat and extended way down at least an octave lower in the transmission line cabinet.
It is such a winning design I have to think it isn't used more because of size, construction cost, and weight. Mine were twice the size and three times the weight of the JBL, and that was just for the woofer. They were so good however I was able to put some little car speakers on top of them and this combo was actually a pretty normal floor stander size and sounded far, far better than the JBL.
Again, this was 1980. More than 40 years ago. It will be a travesty of short-sighted cost-cutting if the ones you just bought aren't light years better. And these were already pretty damn good!