free air subwoofers are horrible most of the time. just throw a box in the back that's easy to remove if it's that much of a concern.

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2006 Audi A3 2.0t DSG apr stage 2
2002 BMW X5 4.4
1997 BMW 328ic sport. moreagrun metallic
1989 BMW 325is with / M50b25nv swap