Thomas has an unenviable position.
He is trying to popularise an unusal subwoofer configuration. The IB has very specific driver requirements and mimimum quantities of the same to function well and survive heavy use. It requires a suitable rear volume for the backwash from all those drivers all flapping in an out together.
Thomas provides extensive and liberally illustrated FAQs with clever and humerous chapter titles that just keep on growing in all directions. I knew I shouldn't have mentioned a book on IBs to him.
Yet the same basic questions (for him) still crop up again and again. Just like any other online forum I suppose.
The basic IB rules may be simple for guaranteed success but these rules aren't written down as simple formulae or tables. Nor are all people's rooms and available rear enclosure spaces the same. So basically you do as Thomas says and enjoy. Or suffer the consequences.
His gods are being very unkind and have suddenly produced a famine of affordable, quality 15" & 18" (IB suitable) drivers.
Just as he set up a new specialist forum to sell the idea of IBs to the unwashed the most popular $100 driver disappeared from the market. (The AEIB15 was an absolute bargain for 4 or more)
With so many people now scratching around for multiiple examples of any suitable and affordable driver I can well understand their own and Thomas' frustration.
8 x $100 drivers + BFD + Pro-amp is a bargain for the bass sound quality and quantity on offer.
8 x $200 + extras begins to look expensive when the results may be in doubt in the builder's mind. Better just order an SVS instead?
8 x $3-400 + extras buys a posh commercial box sub to put in the corner and forget. The Velodyne DD18 or even the big Genelec suddenly look more affordable.
Thomas would argue that the sound quality of the IB beats anything else into a pulp cone. He may well be right.