Adding Sound Dampening

I ordered up 70 sq ft of that brown bread. I read somewhere that adding two more subs to an existing two will add roughly a 3 db increase. Someone also said that 3 db increase is twice as loud to the human ear. I added 2 more Jl 12's to my existing system. It doesnt sound any louder. The key differences are that I am firing all subs down rather than towards me. The enclosure is closer to JL's recomendations. In fact the volume is damn near. The subs are firing directly onto bare metal at the moment. My question is what are the chances this brown bread will increase my spl twice? I know that adding sound dampening will improve the spl and I heard it also imporves the sq. I am just wondering if I can count on being happy with buying this brown bread?

Another thing I noticed is that its not that loud outside the truck either. This makes me happy although I wonder why its happening. I had 3 12's which were made by R/T a few years ago. They were called patriots. Sealed enclosure, about 700 watts or something goin to em, and they were way loud outside and in. Even after I dynamatted they still were hella loud outside. Anyone have any ideas why this is different?

I realize that. well i dont get the whole logarithmic deal, but I know I cant add on x number of drivers to the same power supply and expect to get a increase in spl. More so what I am looking for is people experience with sound dampening and how well it helped there system and in what ways.   I first had 2 12 jl audio w-3 d4's and one kicker zr 360. I now have 4 jl audio subs all the same, and 2 kicker zr 360's. Basically I have doubled my system.

Adding dampening material unto itself will not incfrease your SPL.  It will simply deaden the acoustical space inside the vehicle, aid in stopping vibrations in the sheet metal, and make the vehicle quieter.  It will improve your SQ, probably noticably, and you may perceive this as an increase in SPL since you will be able to more clearly hear the output from your system.