Your wiring is inadequate. Sure the power cables show 13V once the amps shut down. You only get a drop in voltage when there is curent flowing. Stop the current flow (turn off the amps) and the voltage drop will cease. The 1000/1 needs a 4 ga cable all its own, and me personally I would use 4ga to the 300/4 as well. You should be running 2ga as a minimum from the battery to the distro block and 4 ga from there. Also, just because you sanded to bare metal doesn't mean that you have a good ground. The point that you grounded to might not have a good path to ground itself. You also need to check every connection all along the power wire: in and out of under hood fuse, battery terminal, in and out of the distro block, connections to the amps themselves, etc. Also check the continuity of your underhood fuse. I have heard of the ANL fuses looking good, but being cracked and testing bad.
