I've always thought similarly to what Bill said, in that bass horn must be very large to get under 40Hz and keep response flat. If the installation can be made permanent or semi-permanent, then the horns can be made large enough to reach below that, but for portable stuff, I've sort of concluded that 40Hz was a good lower limit for horns. Seems like between 20Hz and 40Hz, everything is pretty much from direct radiation anyway, whether in a horn cabinet or not. So I like the idea of having direct radiating subs and horns from midbass up, at least for relatively small sound production work like clubs and theaters and, of course, people's homes.

But if you can stipulate that several horns be used, then each one can be made a little smaller and still get the job done. For large-scale setups, that becomes an option. Or if the installation can be made permanents or semi-permanent, then you can really come up with some neat large bass horns. And then there's the reality that 40Hz is a pretty good goal for bottom end, and if bass is solid down there, it generally satisfies completely. 120dB between 40Hz and 80Hz sounds very powerful indeed.