Thicker cable may help. It also may be that you can't get rid of it. There are inline conditioners to get rid of electrical interference but they arent cheap. I've heard of people using a capacitor (like lightening caps) in between battery and amp to decrease noise but i have never tried. Also, make sure cable(speaker, preamp, power) don't run near things that give off emf. Lastly, are you using your factory head unit? I have heard these give alot of noise on aftermarket amps. Since youve already redone the amp and speakers might as well do the head too.