I have a 2006 xc90 2.5t and i was thinking about getting subs. Do you need a special type of subwoofer or amp? I have kicker c5 6.5 inch door speakers but the bass just doesnt do it for me with those. With the factory speakers there was a lot more bass. Kicker not as much, but they have very good sound other than the bass. Any help would be appreciated.