The feedback is probably a grounding problem. I had the same problem with my boat but it was the fuel pump. Make sure the batt. grounds are good.
You can get billet grilles that can be powder coated white. Expensive? that is all relative. We sell the rons custom detail grills.
As far as batteries go you have to remember that more batteries= more play time with out the engine running. If the amps are true 1300 watt amps dont expect too much run time. This also depends on the type age and condition of the batts and the charging system.
there is an adapter in development that will be able to make you kenwood xm ready.
Feel free to call me once you have the specifics on your set up and I would be weilling to give you more detailed info (909)481-8411. Also if the guy installed it himself I would advise you to have it checked out by a professional.
Congrats on the new boat.
Bill