dd subwoofers

Hello there guys. I am a total newb when it comes to building a system. But I'm giving it a shot. I have a 2006 Chevy Silverado extended cab. So far I have installed a Kenwood KDC X997 head unit, Focal integration 6.5" components in front doors, Polk 4x6's in rear doors, and two Polk 8" subs in...