Go to
http://www.linearteam.org and download WinISD Beta. It is in perpetual beta form so it's free, but it works well. I use it and get great results. Plug the Thiele-Small Parameters for the RCF driver into WinISD, and begin playing with box types. You will want a vented box, and for two 18" woofers I would say nothing less than a single 8" port, or two 6.5" ports. I'm not sure, but you might be able to make the RCF subs downfiring, with a solid base (called a plinth) instead of firing directly into the carpet. If you go that route, though, make sure the sub is 6" off the ground so that there is plenty of space for the air to move.
