Getting bass to cover the entire music range from horns requires very large amounts of space. Horn cabinets are gargantuan if they are to extend below 60Hz. To get into subwoofer range, you need dozens of cubic feet to enclose a proper horn.
Large venues utilize horns for bass reproduction. There's nothing better than a well designed horn for producing massive quantities of low distortion bass frequencies.
Don't forget Klipschhorns that cleverly use the corner of the room to define the last horn walls. But even then bass is only rated down to 40 Hz. The development of stereo and cheap amplifier power were the downfall of the K-horn (don't need two 104 dB/w/m speakers at home, they're big/expensive, and when you really think about it few rooms have properly spaced corners side by side that have the 10 feet of solid wall on both sides needed to make them work as intended).
Horn design require pressurizing the air that quickly cause colorations from flexing of the horn walls, which is why plastic/metal mid/treble horns are often dampened or made of solid blocks of wood. When you consider the math and the size needed to achieve 20 Hz output via horn design while addressing the colorations you realize that the only viable solution are garage sized concrete structures. But they're reported to be world beaters (blending/phasing/time delays to mid/tweeter aside). Some creative DIYer's have tried heavy plywood horn construction with expanding foam chambers behind the plywood to dampen/stiffen, but still only a partial coloration solution and frequency response limited by the laws of physics.
