One farad = 1000 watts. Do you have a 5 thousand watt system? Really man, save some money and just get a 2 farad, if you really feel the need for a cap. Do the big 3, if you haven't already, and see what that does for the problems you're having.
Do some reading on caps then come back and see if you really need one. You will find that there are not many fans of caps around here. Nothing makes up for a good install...