You really need to measure amperage, not voltage. You are pulling a lot of amps through that wire, even though it is a short run, I'd recommend stepping up to 1/O power and ground wire (instead of the 2g). Another thing to consider: Are all the amps shutting down at the same time? If so, you might want to take a look at the wiring for the head unit. Do you have a relay on the "turn-on" lead from the radio? If you can try and isolate the head unit power from the amp power.
Good Stuff. It does shut down at the same time. I do have the head unit powered by the AMP batteries. Should I run the head unit off the engine battery? Thanks Check your PM