A couple of friends and I are about to start building 3 Tempest based subs. I downloaded LspCad a while back & have been playing around with it for quite a while. We all decided to go w/ a sonosub design, 300L (just under 11 ft3) internal volume, dual 6ID inch ports 29 inches long, tuned to 18hz. Ok, my questions are, what kind of real world SPL levels can I expect from this guy using a 250 watt amp? Where can I find an inexpensive outboard amp to power these guys? Will I need to use some kind of subsonic filter with these subs to protect the driver in the above configuration? I tried searching both here and hometheatertalk to get answers to these questions, but couldn't really find anything (but I did find a LOT of other good info about subs in general and the Tempest in particular). Thanks in advance.