ok.. I have a 2013 Ford E150 van. I just installed an XS Power D6500 under the hood and an XS Power XP3000 behind the seat. Hooked both batteries up positive to positive and negative to negative. Added a ground from the hood battery to an existing ground under the hood - and added a ground from the back battery to the frame of the van with a grommet. Both grounds wire brushed and shiny. Installed a Sundown Audio SAE 2000d to an X15. When the van is on and running, the voltage is 14.2. When I crank up the volume, the voltage obviously drops. It gets down to about 13.4.. But when I turn down the volume and shut the radio off, the voltage rapidly jumps between 13.4 and 14 volts - like it’s confused or something.. WHAT is the cause for this voltage jumping? Also, when it’s turned up and the voltage is jumping, I can see my sub “burp” or “jump” forward or backwards while it’s playing. That can’t be good on the sub.. Any ideas? Thank you!!