@russbutton Great explanation of horns, but i would add that a 40Hz horn quarter wave is just over 7Ft, so a full-range horn is impractical in most domestic settings.
The Klipchorn 'cheat' of 1/8 space loading the woofer is so room variable that its hard to predict the actual perfomance. In a purpose-built room the LF can perform admirably, but the MF and HF are still mediocre compared to other higher quality horns.
Where our opinions diverge is on Transmission Lines. Properly calculated and damped they are capable of superb bass. Yes, they give up the backwave reinforcement and so are lower in efficiency, but unless you are designing a low-power or max output system, that's not much of an issue. They are larger and more complex (expensive) to build than a traditional ported or sealed design which accounts for much/most of their lack of popularity commercially, but relative popularity for DIY projects.