I know that the speakers in the door (I think they are 4") are almost impossible to replace because most speakers won't fit. The stockers are very shallow. I've been told that you can upgrade the tweeter and add new/better crossovers to the existing mids, and have all of that powered with a new amp.

I'm still on the fence with upgrading my system. But good luck with yours

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*2010 X5d - Space Gray/Black Nevada/Dark Bamboo|ZPP|ZAP|ZCW|ZTP|ZRC|322|328|386|4NC|4UB|655 |6FL
~Ordered 8/27; Production began 9/28 at 12:20pm; Completed 10/7; Delivered 11/4!~
*2008 535i Black/Black - Dinan Exhaust & S2 Software
*1995 318ti Clubsport - Hellrot Red (autocross/fun car!)
Past: 2005 X3 3.0i; 2009 335xi