Dimming lights

Hi, this is my first post! My english is no good but I would try to communicate with you guys. My problem is that my car (1.6L) with only a 70amp fuse and just 1 60amp amplifier(mono block) is making all ,my lights to dimmed when the bass hits. Even with no a/c on, the voltage drop is considerable, very noticeable. My battery is a Optimad Red Top and I add 4 gauge cables from alt to bat(fused) and from bat to chasis, I call it the big 2 because the engine block ground is to hard to take it out and put a second cable in the same grounding point. What else could I do to eliminate the voltage drop and don't have to heard the music at low volume? Thanks and sorry 4 my english