Transmission line speakers!

That’s a really impressive TL cabinet oldhvymec.

Transmission line woofer enclosures are a way of effectively doubling the size of the woofer by turning it's back wave around and getting it to come out in phase with the front wave at the "right" frequencies. It is a very difficult design to get right and requires a lot of trial and error fiddling. It is best to keep the woofer under 200 Hz or too much coloration will get into the midrange. Done right it can make a smallish loudspeaker much more powerful. The best one I have heard is in Roger Sander's Model 10 ESL. He bi amps them and uses a dBx Driverack as a crossover. 

I had a pair of TylerAcoustics HT2-TL speakers that I could not get any 80hz bass out of, in multiple rooms with multiple front ends.  Beautiful looking speakers, smooth, detailed and refined sound, but my Fritz Carrera BE’s smashed them in bass response.  I was able to get the Carreras to play -3db at 20hz and pretty much flat from 23hz to 100hz.

I had a pair of Clements Audio RT-7 that I used for years. Great speaker. 2-Way with a true ribbon tweeter and a small transmission-line loaded woofer. Narrow baffle. Nice sounding speaker. I auditioned it against a comparably sized and priced Ohm Omni speaker at the time and chose the Clements.

I may still be using them today if the cosmetic/aesthetic were better. The early RT-7s had a foam wrap around the whole column and the column simply sat in a base. The speaker column had a beveled edge on the bottom and the base had an opposite beveled edge to accept the speaker column. Wasn't particularly stable or instill a great amount of confidence in the junction between speaker column and base.

Later versions of the Clements Audio RT-7 did away with the column/base setup and also changed the finish from a foam wrap to finished hardwood veneer. 

Transmission lines are still ported speakers, although when properly designed tend to sound a bit better. Still nothing compares to a properly designed acoustic suspension design to my ears.