Lets assume that we live in a perfect world and every vehicle came stock with say a 500 amp alt and 2 or 3 Kinetik batteries. If you had 1 solo x 18 running with 5000rms and hit a certain number on the meter how much would the number jump if you added 1 more solo x and 5000 more watts. That is assuming that you had sufficent electrical and a box big enough for both subs.
6 db is kinda what i have seen. at least a good guess if its done right.... but really there is no way to tell. shape and resence of the car and all that junk changings everything.
i have been told double the power adds 3db doubleing everything adds 6