I want to say that the gauge of wire is too small, I would go with 4 or better, or at least 6.
Here's an article about the gauge.
Also I have seen in many dual amplifier setups people using a power distributor, using a 4 gauge wire from the battery, in your case the red top, going to a distributor, then moving from a 4 gauge to maybe a 6 gauge from the distributor to each of the amplifiers. Grounding is definitely important as well, I'm not too sure on what the best ground is in the suby, maybe the trunk latch? I'm curious as well as I'm working to combat this situation as well, I just ordered a 160 watt amplifier from Maniac. Hopefully this will resolve the issue. Also lowering your gain on the amplifier may help as well until the issue is identified.