Transmission line speakers!

They use a passive radiator on the bottom to tune to box.. Still a port.. I've tuned these with putty, add and subtract the mass on the bottom.. Quite effective per room and amp.. (Q)uality tuning.

My Infinity Monitor IIa’s are transmission line. I bought them new in 1977 and still play them daily. They are rated flat down to 22hz and have the best bass performance (ie, most pleasing to me) of any speaker I have had - including my Von Schweikert VR4-III’s, which are also transmission line (per Albert Von Schweikert).

Some of Rega’s  speakers are purportedly TL.

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.