I'm all for the theoretical advantages of multiples operating in their linear envelope. Even though the better motor topologies are clearly superior, there's is more to a subwoofer than linear BL/excursion. Suspension linearity, total operational noises, resonances, and dynamic compression.... all contribute significantly to the overall picture.
I know I'm not stating anything new, however the beauty of the addressing the lower octaves is the large wavelength size. These long dimensions allow us to employ multiple drivers so that all the distortion products associated with high excursion levels are minimized.
Nice score on the IB-15s. What is going on with AE's stock issues? Is it his suppliers/build houses? The current choices for IB builds sure seems limited to some of the higher priced options.