Turn the gain on your sub amp down and then unhook the rca's. Then tune your components to play as clean and as loud as they can, probably have a high pass on them of 60hz to 80hz. Then hook up your sub again and tune it to sound right with the rest of your setup.
IMO if you don't tune what you have and add more speakers all you will really being doing is spending money to add more noise to your system, not music.
If you can't get your components as loud and clean as you want them then you probably need to add more clean power to them.