OK here's some of the rules of thumb as I know it. You need to double your amp power to gain 3 dB, and a 10 dB increase is perceived as twice as loud. Is that right? Now, I've heard another way to get the 3 dB increase is to double your cone size. In other words, 2 12" woofers would put out 3 dB more than one. My main question is, if the above theory is correct, can you get that same 3 dB increase by just adding 1 sub, and keep the same amount of power (split between the 2 subs)? Or in addition to adding one more sub, do you need to supply it with the same power as the other?