Transmission line speakers!

I started designing Transmission line speakers in the mid '70's. They can be real good for the money. These days I use a ported cabinet design because you can get a better final performance curve.

Though not a speaker designer, this meets with my experience. 

I had  pair of Spendor D7 which uses a quasi-transmission line bottom firing port.  The bass and sub-bass was so deep and effortless, a speaker with very special bass.  It improves the efficiency of bass response and makes them easier to position. 

No, I don't believe the Spendor D7 or D7.2 even qualify as "quasi-transmission line" (whatever that means). They are simply bottom ported. 

I had a pair of IMF TLS 80s in the mid-70s and loved the bass they produced. However, I found the midrange quite poor and they did not image very well. I sold them for a pair of Braun L810s - bass not as deep as the IMF but the midrange was far superior. 

My Acoustic Zen Crescendo II’s are a transmission line design...Best sounding loudspeakers I have ever heard or owned.

Nice demo!

Khorn. A transmission line with a fold or two. Just my interpretation, as I am no speaker designer. PWK could have ported it, but would not get the efficiency of a true horn, nor the quality of output. Transmission loaded, ime, always had folds within the cabinets, otherwise, it was simply ported.

The K-Horn does have some folds, but is not functioning as a true 1/4 wave T-Line. It’s more like a wave guide or horn to help amplify certain ranges of the bass for efficiency gains, as opposed to absorbing resonance and allowing deeper bass from a given driver. Very different principles with different purpose and execution of the folds.

...as opposed to absorbing resonance and allowing deeper bass from a given driver. 

Bingo.