Dividing Fs by Qts, we want greater than or equal to 120. The EVM 12L comes in at 237. The JBL Prosound division have a driver for bass applications along with some theory on horns at Technical Notes . Other good horn theory resides in scientific papers and magazine articles, see the References section. Horn Size and Placement Horn loudspeakers can be roughly divided up into bass, midrange, and tweeter horns.
Tweeter horns are very small and front-facing with a straight horn axis.
See for example the Avantgarde horns. They are usually front-facing with a straight axis, though the larger ones can have a bend in the axis. Ed Billeci's DIY page features this very cool 80 Hz front horn . Bass horns have to be long to work and require large mouths. The true benefits from horns of acoustic impedance matching do not occur until you reach at least a quarter wavelength of your lowest frequency, and more likely a half wavelength.