I was at a GTG six months ago in Austin where the owner had an IB setup using dual 15's on opposite side of his "master bedroom size" HT and then last weekend I flew to Deleware to attend a GTG of a buddy who put a SVS PB13 Ultra in a huge "very professional" looking system with four rows of teired seating and Top fo the Line Dynaudio speakers all the way around. Both systems were fantastic for what they did.
The IB was far better at pushing more air (and you felt it) but don't discount the PB13 because that gave some of the most realstic impressive bass in a huge theater I have ever heard. The benefit of the PB13 was the fact that you could control all aspects of the sub, while with an IB setup, the bass gets lower but the back pressure can be heard by nearby neighbors if you play the system loud enough.
Because you are undecided, I would look more towards the SVS PB13 for better user control. If you feel your house is far enough away then go with the IB. A good test if you can do it, is to hook up your most powerful sub in the attic, turn it up very loud (not enough to harm the sub) and play some bass heavy demo scenes. Step outside and walk till you can no longer hear that. Now when you reach the distance where you can't hear it. Double that distance and you should be safe enough to try an IB setup.
There are some great videos of IB's in use on Youtube and there is one where a guy is in a huge closet like area behind his subs and the bass pressure is insane with how powerful the back pressure from the subs become (he used 4 15"s) It can become very insane, but you have to ask yourself is more better? I have a SVS NSD12/2 in my great room and although would love the PB13 the current sub I'm using is only level 3 (out of 10) so since I barely have this on, getting a sub so powerful would have no effect, because I'm not even coming close to using the current one to it's potential.
