You do realize that with the Monsoon system the 6x9 is actually a subwoofer, with a smaller 3.5" full range driver nested within the 6x9. Now with that being said you could replace just the subwoofer if you wanted and have nothing but bass in the back
WoofersEtc.com - 6x9" Subwoofers
or you can replace with a full range 6x9 coaxial speaker of your choosing but then you have to do some rewiring, or you can replace with OEM type speaker 2006 PONTIAC GRAND PRIX Parts - GM Parts | GM Accessories | Shop GM Parts
the call is yours to make just my 2 cents worth
