[SteveCallas]

Sonnie, I also don't think it's a good idea to try and utilize both. Every now and then I read of people wanting to try to combine subs so that one is responsible for upper bass and one just for the really low end, and to me it just doesn't make any sense. The subs that can handle the really low end great (without using any EQ tricks) are usually monster subwoofers that will also be able to handle the upper bass superbly 99% of the time. The upper bass would just be a complete cake walk for them. In this case, with good drivers, you'd benefit more from just letting the IB handle it all. In addition, I'm not a big fan of the idea of using a ported sub tuned too much higher then ~18hz.

If you really want to pursue the IB, and you know of a spot for mounting where you are sure you will get response at least as good as your PB12/Plus2's, I'd sell them and buy more drivers for the IB.