While you can certainly ask your questions here, you may want to look in the dedicated board for sound systems/electronics, on this site. Lots of posts there about what amps to use, what speakers have been used, and so on.

BSW's Mobile Electronics Forum - Xoutpost.com

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue