I was visiting the Madison Speakers website just now, and followed their link to the "Tech Page". It's hosted on another site, and they say it's a "collaboration". http://quadrasub.netfirms.com/ Well, I really hope the guys at Madison know more than whoever it was that put together this page - which was somebody who doesn't understand much about how speakers work. Actually, no -- the SPL does not increase. What happens is that the Vas parameter for the two speakers combined is half of what it was for one speaker, allowing the same alignment in a box half the size. And, they can be sealed or vented. This is about as confusing an explanation, if not quite "wrong", of a bandbass box as you can get. A bandpass design is essentially normal speaker box (usually sealed), with an "acoustic crossover" -- another, vented box -- placed over the front of the woofers. First, wrong. Second, a gross oversimplification. Useless, and dumb. I stopped reading here.