My mains speakers are some transmission line IMF RSPM mark IV.
I bought them in great condition from a junk shop a year or so ago.
My magnepan .7's and Dalhquist dqm9 have been pushed into a corner ever since.
The IMF's are huge and heavy but are so easy on the ear.
I am sure that design difficulty, cost of manufacture and shipping, and shear size are the reasons speaker like this became rarities.