Subwoofer upgrade and an amp to drive it. Sounds heavenly.

MB Quart DS1-254 Discus 10 Inch 4 Ohm DVC Shallow Subwoofer Dual 4 ohm 2 inch voice coils, 200W RMS
https://mbquart.com/shop/mobile/mobi...low-subwoofer/

MB Quart RA1-300.1 Reference 300 Watt Mono Block Amplifier with Bass Remote, Amp Power @ 2 Ohms 1 x 325W
https://mbquart.com/shop/mobile/mobi...att-amplifier/

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'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.