I've been looking into having an IB, too. I've been following the IB forum for several months and it is a great source of info. One thing that they continually say is there is no replacement for displacement. In light of that, I think you will most likely need more than 4 x 12" subwoofers to avoid bottoming out. Even though Danny's subs look impressive, I'd be very wary of having too much power driving them. I would be very hesitant to have more than 100 watts going to each sub if you do end up going with an IB design. I'm somewhat of a novice, but I would think 8 would actually be the minimum, and even more would be better. It sounds like overkill, but when you think about it, there is very little pressure holding the cone in place, so a surprisingly small amount of power will get them moving a considerable amount. As with a line array, the more speakers you have, the less each one moves, so at a desired volume level, you need more to keep each from moving too much.
