Checked with Best Buy and they can put a 10" sub in a box, with Alpine Amp hidden away in the side compartment for about 500 bucks. Other than needing more base I am happy with the factory speakers. They only issue is the pre-out location. If they cant find it they will use a rear speaker signal and use that for the sub! Not sure how good a signal thats going to be. Anyone know if there is a sub pre-out anywhere back there for a 2003 non-DSP car!?
Thanks

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2003 BMW X5 3.0i Steptronic, Sport, Prem, Rear Climate, Xenon