Planning to install an AMP/Subwoofer setup in my E46 M3, suggestions? I dont know enough about amps and subwoofers, can someone give me some pointers? I am planning to keep the existing Harmon Kardon speakers. What type of amps or subwoofers should i get? Planning to get these at Bestbuy, i know they may not have the high end stuffs, but i got a $300 gift card for my bday and planning to add money on top of course. Any suggestions?