A speaker with well-designed transmission-line woofer loading is the ESS Transtatic I, from the very-early 70's (I first saw and heard it in 1971). The woofer in that speaker was the famous KEF B139, a great driver for it's time (David Wilson used two of them as the mid-bass driver in his WAMM loudspeaker).
The reason ESS ended up being the manufacturer of the Heil AMT speaker is that Dr. Heil had heard the Transtatic, and wanted to use the B139 in the speaker he was designing. He came into the hi-fi specialty shop in San Jose that was an ESS retailer, TV-Audio Center, Inc. (where I heard the speaker, and where legendary High End retailer John Garland worker at the time), to find out where he could buy some of the drivers. They put him in contact with ESS, and the rest is history. I have a pair of Transtatics; if you're anywhere close to the Portland, Oregon area, come on by!