My experiences with subs has been limited to car audio. I just remember how pleased I was with JL's. I've searched this board on sonotube threads and have learned a lot. I noticed the downfiring issue in the threads. I'll probably go with the Shiva. I like the sonosube at this site: http://www.extremezone.com/~stpeters/subwoof.html
Right now I have a Polk PSW450. It shakes the cabinents but I want ~20hz range, I want to feel it more. Thanks for the advice so far.
Any woofers will work in a sonosub design. The same theories apply despite the driver, unlike a T-Line which needs the right driver. I ran a 15w3 in a 5cu. ft. ported box with a 250 watts plate amp in my house. I built it for my friend's car, the thing shook the house violently at the right frequencies. But the woofers are way too inefficient for practical use in a house.