I'd take Sonnie's suggestion and go one step further. Do what he describes, but don't limit yourself to only where an IB can go. You might find that the best potential response comes from only where a box can go. You might also find the best response comes from only where an IB can go. If the best response comes from a place both can go, then you have to figure out which type of bass you want. I have a two 18" drive IB, but want to double 'em up to four. I would expect you to need four of those 15" drivers to be happy (as four of them have only slightly more output than my two 18's). This is just some food for thought. Good luck!