Vista - How to turn on my subwoofer

I have recently done a home made sub setup with a 600 watt psu and 600 watt amp witha 10inch kicker sitting in my floor i have the headphone to rca adapter connected to my amp and when i plug it into the normal sound port is blasts and shakes the house but i cant seem to enable it without doing it like that i got 5.1 built into my motherboard and ive messed with so many settins to try and get it to work it just wont turn on for me anyone have any advice im running I7 6 gigs of ram 4870x2 1tb hd realtek soundchip please help me