Ref the video, these clips are Paul's "Dear Abby" spot where he gives a quick answer to complex problems. So I don't fault him for not giving a more complete answer. In regards to the question I would note that's there's theory where budgets are unlimited, and actually building a speaker you can sell. Usually the largest cost factor is the cabinet so if you're bracing an entire speaker cabinet to handle a 12" woofer or more commonly, building a subwoofer inside of a speaker cabinet, it gets expensive very quickly. There are numerous manufacturers that do this and some are mentioned on this thread, but they are all expensive products. It's cheaper to get good bass with multiple smaller woofers, and again as previously noted here, it's what the market wants.