if you want the simple way without getting technical. set the volume at about 3/4. turn the gain(input level) on the amps all the way down. now with a nice clear cd that you would typically listen to turn the gains up until the subs just distort, then turn it down a bit. this will get you relatively close to where you need to be. there are much more involved ways which you can explore but i dont know how serious you are about car audio. this method will suffice. it is a knock off way of doing it but it will be good enough to help the amp from clipping and the subs from bottoming out.