Hello mcraghead. Transmission lines taper from larger to smaller while horns taper from smaller to wider, the opposite. There are mathematics dictating the rate of taper. The transmission line loads the rear of the speaker, and tends to damp resonances. Horns allow the speaker's cone to move freely, very effectively coupled to the room. Several companies produce transmission lines without talking much about them. The foremost advocate of transmission lines, Bud Fried, produced speakers which have been copied by others who have been successful at selling them. I encourage you to build a transmission line enclosure. Be very careful to get the dimensions right and be sure to choose the right driver. The narrow front, deep cabinet using multiple small diameter woofers is ideal for a transmission line. The design is more difficult to build than an bass reflex cabinet, but the effort will be worth it. A sealed enclosure of the right dimensions, paired with the right driver, can be very easy to make and quite effective. Enjoy the music.