My headlights don't dim where as the rest of my lights do, pretty greatly, on my 1000watt amp. 4GA wire with a 120amp fuse, amp is fused at 80amps. But even my old amp with the 25A fuse did that as well.

Big 3 upgrade, that should fix it.

In my case my car is so power-f*cked that I don't think I'll ever NOT have issues. -_-