I'm a little late to the party, but here is my practical and theoretical experience with Transmission Line (TL) loudspeakers:
1. Almost all TL loudspeakers which have been designed without George L. Augspurger's TLworks, or Martin J. King's TL MathCad worksheets, or David McBean's HornResp, are plain wrong.
2. All TL loudspeakers measured so far in Stereophile magazine and SoundStage are plain wrong.
3. How can you recognize if the TL design is wrong? By two tell-tales signs:
a) Frequency response has big notch (or depression) at about 150 - 200 Hz (lower in bigger boxes). Also, frequency response in smaller enclosures is not extended toward low bass, in contrast to vented enclosures with similar volume and efficiency.
b) Impedance has two prominent peaks in the bass (like bass-reflex box).
4. Although "theoretical" definition for TL is "infinite long pipe which absorbs woofer's back wave", the practical definition for TL is something like this: "vented enclosure with quarter wavelength resonance (long channel/line) at, or lower than woofer own resonant frequency". This means that the output from the TL port should contribute the woofer output. TL channel/line length is about ¼ wavelength of cut-off frequency.
5. Mass-loaded TL (ML TL) is a different beast - it is bass-reflex box with TL quarter wavelength resonance, or TL box with restricted terminus. We can skip it for now.
6. Optimal TL design has woofer placed not in the beginning of TL channel, but with some offset. Classic Bailey TL (from 1965) and modern PMC TLs have woofers at the beginning of the line and therefore are not optimal designs.
7. The optimal TL impedance should have depressed lower (first) impedance peak (or completely flat), leaving only the second impedance peak higher in the bass. It is mostly stuffing/damping issue.
8. Optimally designed TL enclosure has tight, punchy bass – even with high Qts woofers. Enclosure volume is moderate to very big. Low cut-off frequency is higher than a vented box with the same driver, but lower than in closed box. All-in-all, not worth the pain, except for moderate to high Qts woofers.