Any 2008-2009 Tundra Double-Cab Owners Have An Opinion On Upgrading Audio System In Truck. I Have The Stock Audio System In Place; Which Is The AM/FM/6 Disc CD Changer/With XM Satellite Radio Module (Not JBL System) I Feel That The Radio/Head Unit Is Probably OK. However Thinking Of Replacing the Factory Amplifier and the Original Speakers. Have Read Several Posts, But Not Clear On Factory Speaker Size, And/Or What Works Best For Replacement Speakers. Also Plan To Add Sub-Woofer. I Know That Limited Has JBL System, Which Includes Sub-Woofer. Where Is The Sub-Woofer Mounted On Limited? Any Thoughts On This Prospective Upgrade Will Be Greatly Appreciated.