that's just freaky. How do you sound proof an IB I wonder? I totally have the ideal situation for one, but worry about keeping the fiancee up at night, or the neighbors, or you know, other countries....
That's how it's supposed to be! There is a reason companies keep introducing louder, lower and cleaner subwoofers.
To put that in perspective, if the above mentioned system was reasonably well confined, another pair of Avalanche 18s could be well justified if the system was measured and EQ'd to extend into the 10-14Hz range and -5dB to 0dB playback levels were desired. Of course I would certainly wager that the owner of the above mentioned IB first needs to upgrade his mains to put any more sub-bass energy to appropriate use.