At the risk of drawing out a thread that's already near the breaking point I am going to weigh in here.....

One benefit of the using multiple subs that I don't think has been mentioned here is that by doubling or quadrupling the number of drivers, the excursion required to reproduced a given frequency and SPL, the excursion of each is greatly reduced. This reduces several types of distortion, and is the basic principle behind line arrays. If the burden of generating absolute SPL is reduced for individual drivers, the fidelity will be increased. This is one reason quality headphones are so accurate. So in this way at least, two or more cheaper drivers can produce a qualitative and not just quantitative improvement.