Hi Bob
You should choose your drivers with care for an IB.
You want a decent Xmax to allow you to relax and enjoy any reasonable SPLS at any frequency. 15"drivers are almost the norm with 18" becoming more popular all the time. With 12" drivers you need a lot to give you decent headroom.
Check out the list of recommended drivers at The IB Cult where they are listed in dollars against displacement order. This makes sense with an IB where displacement is important for headroom at very low frequencies.
There are two lists. One is more up to date than the other. Both lists show discontinued drivers as some drivers might still be available on the used market.
Also do check out ThomasW's great FAQs on the IB Cult.
http://ibsubwoofers.proboards51.com/index.cgi?