Wayne's post is, as usual, right on.
The critical thing is that it's relatively easy to get 100 db eff over 150 hz. Below that it starts getting difficult.
Using subs is one suggestion. Here are a couple of other practical ones (of course you could always blow pots of money and buy a giant bass horn that is currently manufactured and sold but the ideas below are cheaper):
One thing you may want to consider is to buy a used klipschorn and tinker around with the drivers/midhorn/tweeter section, but use the bass horn. This should get you bass down to 40 hz for sure, if not a bit lower, with eff to over 100 db.
Plus used bass horns that are not working/trashed out are pretty cheap: around a grand at most.
Another option for you to consider is the old Altec VOTT design, that gave bass down to about 50 hz with about 99 db eff (if I remember correctly). This is a combination BR plus front loaded horn.
you could do a 3 way with this set up that will be great. VOTT boxes are a few hundred each at most.