Transmission lines have the reputation of excellent deep bass, but they are also very, very inefficient. They need a big amp.
Iʻm not fond of those transmission line designs that have only an 8" bass driver, or even two of them. You need surface area to move a lot of air. Thereʻs no replacement for displacement.
Just because you donʻt have the tools to build from scratch doesnʻt mean you canʻt do a project. Go to any local cabinet shop with a diagram of what you want and they can cut the parts for you. They could also assemble the enclosure for you if you need it. It wonʻt cost as much as you might think.
As for horns, the enclosure stops acting like a horn based upon the size of the horn mouth. The lowest frequency that it acts like a horn is based upon the quarter wave length. If your horn mouth is 3 feet across, then that quarter wave length is:
The speed of sound
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full wave length
The speed of sound is 1100 ft/second. If the horn mouth is 3 feet, then the wave length is 12 feet. 1100/12 = 91 hz. The reason that Klipsch made corner horns is that they use the room corner as part of the horn. Thatʻs how you get a horn mouth that is 8 feet across, which is the quarter wave length at about 34 hz.