Edit: SPL/energy is indeed doubled from 112 dB SPL to 115 dB SPL. Use of "twice as loud" and "twice the SPL" interchangeably on John's post threw me off.

This paper that John (jakeman) has pm'd me a while back was quite interesting. Four subs is perhaps the way to go if one has the room, funds, and WAF in their favor.

John

John
Our HT