But if you can't use corners, then the horn you need becomes very large. If that's an option for you, then something like the ten π works very well. But even that is tuned to be used above 40Hz and in constrained space or in multiples. That's the problem - to get down into the bottom octave, you just really need size. Most guys running horn subs add EQ for the bottom octave, which goes to show that they aren't acting as horns down there. That means that - for the bottom octave - they're direct radiators in fancy boxes. This is true for all the popular basshorns discussed on these forums.

But this is probably a debate about semantics. There are lots of people that really like the power a large-format horn can deliver down to 40Hz. I'd suggest the ten π might work very well for you. They deliver solid output down to 40Hz, and they are able to deliver extended response as well. This makes your choice of crossover much easier - You can cross them low or use them up fairly high. The rising response of the driver combined with the diagonal reflectors allows smooth response at high frequencies, almost to the limits of the driver, itself.