volume doubles every 10 decibles, every time you double the power to yours system it adds 3 decibles. in short the single 10 in that may add another 500 rms tops. this will not make it that much louder it will only enhance the high bass around 150-200 hz by about 1 db this is why there is no point in adding a 10" when you already have 2 15" it will just take up more space to add maybe 1-2 more db
