engine noise easy to remove, its just sound from ur altinator, got strathfiel or JBHIFI and ask for a sound filter for your RCA leads, will get rid of all unwanted sounds and interferance

The only reason i can see you goin for 6x9's is for extra bass, but you will get heaps better bass/mid from a small subwoofer in a sealed box. when u install the sub, sell the 6x9's, and replace them with a small coax if you really feel the need for rear fill. Bass Cancellation can also occur from mixing 6x9s with subs when the 6x9 is installed as a free air application in your boot form the tray.