Aftermarket Radio with Factory Subs (Shaker 500)

[Wolfman064]

I've run searches and can't find the answer, although I know this has been discussed. After replacing my Shaker 500 system with an aftermarket unit, and hooking up the door subwoofers, I now get a "pop" out of them every time the car starts. I think there was discussion some time ago about putting in either some sort of crossover or line filter to get rid of this... Anyone know what I can do to fix it? Thanks!