I was always under the impression that it was best to ground to the frame in the shortest distance possible?
But your saying if I ground the cap and amp to the 2nd battery in the back I may pick up a little? I am not going to run the ground for the 2nd battery all the way to the front battery, as I had a hard enough time running the first wire where I wanted it. I think I may upgrade the ground wires up front and move the rear amp and cap grounds to the 2nd battery and see, as I am getting a little clipping.
I have a question though, on only one particular song, a bass comp song with very low notes, the amp will go into protect mode, but I was watching the cap's voltage readout, and it never dropped below 12 something volts. Could this be because of grounds?