boat system set-up and voltage issue

Hi guys, need a little advice as I am getting irritated with my boat set-up..It's simple yet effective.

I'm having an issue with voltage drop off in my Crownline.

I run two Dual Purpose batts..10amp 5/5 each bank on board charger.Alpine HU running 4 speakers and a Kicker ZR360 running a 12" Kicker in a ported box...Not excessive at all...4 gauge from both batts.Off the 4 ga I run the HU and the amp.Prior I ran the HU off the boat harness but did not like the common speaker ground set-up.I since ran my own wires and power and used a common ground bus bar off the boat to ground the HU and the amp.All terminals are good and the batts are fresh marine Dual purpose 1000's..

Here is what is wizing me off....Key off and I can check the voltage at the batts and at the amp/HU lead and it is 13.2-13.5..somewhere in there.Turn just the HU on and play for a few minutes all is well..Turn the HU up and add some bass into it(without) the sub hooked up and the lights will start to dim on the deck.Turn it up some more past that and it will clip the HU off...very annoying!.I can't imagine what will happen when the amp is actually driving the sub..WTF is going on here..The whole time the charger is on and the shore power is on.

Do I need another batt? Maybe a 3rd
Do I need another HU?..seems to be killing the voltage maybe?..works perfectly just like last year