If there was an 8" woofer or any thing else that was a better solution that what I did use with the OB-5 and OB-7 designs then I'd use it too.
What I found is that crossover point is in the lower vocal region and splitting the signal up into dissimilar drivers, with different voice coil off sets, causes a phase shift (unless you are controlling it actively like ebag4 did) and looses coherency. A lower crossover point helps but about 200Hz is as low as a pair of M-130/16's can go in free air.
Plus allowing the low frequency load to be shared with four smaller woofers (OB-7 version) lowered the distortion by lowering the load to each driver, lowered the exertion level of each driver (less stored energy due to inertia) and lowered the mass compared to a larger woofer (less mass to have to put into motion and stop).
So the use of more smaller drivers did have its advantages.
Plus, the sealed box design (OB-7) kept bass response really tight and clean.
It is a well proven and tested design too.
For what it's worth I have some new 12" woofers on the way that are designed to play in free air and are servo controlled. You can read about those in my circle.
