not to sound like a sales person..but check the BSW's Mobile Electronics section.. they discuss this in more detail..

there are a few options.... one is obviously to go with bigger, badder sound system that requires modification to your vehicle..(which will also void warrantee)

the second, and more popular option is swapping out the stock speakers for better ones of the similar size.....

Polk Audio or MB Quarts are popular alternatives...in addition to the BSW Stage 1 speaker upgrade kit that will be available for the E70 soon!!

I personally am considering MB Quarts for the doors and dash....and plan on leaving the stock amp alone...(I believe this to be the less expensive and more effective option)

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2012 X5 50i M-Sport
2014 X3 35i M-Sport