I've had an IB setup for about 3 weeks now. I'm running 4 Adire Audio Tempest drivers and 1400 watts. It is absolutely incredible.
One of the reasons for the multiple drivers is that most drivers are more linear in the first x percent of cone movement. Each driver you add results in all drivers having to move less for the same overall SPL. This also results in more accurate sound at that SPL level.
When building an IB, a primary goal should be to make sure that any "throat" or manifold has no effect at all on the sound. I believe the FAQ over on the Cult forum talks about proper sizing if a manifold is used. It's pretty simple.
If you're trying to figure out what the "trick" is with IB subs, you're probably making it too complicated. The beauty is in the simplicity. In my opinion, it's about as much the opposite of a bandpass design as you can get. Many, if not most, bandpass designs sacrifice sound quality and a wide bandwidth for higher SPL over a narrow range of frequencies and maybe a smaller enclosure.
IB subs sacrifice some SPL for sound quality and a frequency range that extends well into subsonic levels. Which is another reason for multiple drivers. You get the SPL levels you need and keep the sound quality and frequency range.
-Chris
Oh, I meant to post that I've measured over 110 DB on my Radio Shack SPL meter, uncorrected, at 18Hz. That's about 119 DB corrected. There wasn't a hint of distress at that level and I could have turned it up more though I'm not sure how much. Just wasn't sure my house could take it.