Transmission line speakers!

   All PMC speakers use transmission line designs.  Consequently, they seem to produce more and better bass than comparably sized speakers using other designs. 
   I really like my PMC Fb1i Signature speakers.  The only real knock on them I’ve ever heard is complaints about the cost, but apparently it’s not that easy to design and build good transmission line speakers.  I haven’t encountered any small 2-way towers that are better, certainly not in every parameter.  After a certain point, everything in engineering is a trade-off. 

I owned the model right below the TDL reference speakers twice. I sold the first pair under duress because of their size and always regretted it. Quite a few years later when I finally had a dedicated listening room and had been through a number of other speakers, large and small, I found another used pair online and traveled a couple of hundred miles to buy them. I had them for a very short while when I read some glowing things about Klipsch Epic CF-4 version 1 speakers. I bought them on a guess, and suddenly the TDL's didn't sound good anymore. The Epics were so much more coherent. I sold them at a loss and never looked back. 

Sometimes it's true that you "can't go home". 

Transmission line use a long internal pathway. for the bass and are ported as I recall. IMF (Irving M Fried) were doing this back in the late 1980's and their floor standing speaker did indeed have deep, articulate bass. I think the design usually results in a low efficiency speaker. The few times I've owned or heard these types I always liked the bass but often it seemed somewhat disconnected from the mid and highs - almost as if the bass were too goo or too present for the rest of the speaker if that makes sense. Worth exploring though.