At 15Hz, you'll get room gain at 9 feet out. Move it to 10 feet out, and according to what I read Mark Seaton post, you won't get room gain at 15Hz anymore. Although I imagine +30dB is a lot of room gain. Much more than I would think of, but then I don't really know what formulas to use for room gain. How are your IB subs installed? It might just be that the position and installation architecture doesn't lend itself to room gain in the same way.
This is the waterfall chart for F'ing Irene:
waterfall. Maybe that was the background beat you heard. I'm not really hearing a lot in terms of a bass line during this scene myself. What I do for Black Hawk Down is to set it so the average volume is around 80dB. Otherwise I'm afraid of damaging my hearing.
Theoretically, an IB of sufficient power and driver volume/capabilities should be better than the 16-46PCi across all frequencies, including low frequencies.