so it seems like a reasonable yet effective solution could be opting for the premium sound pkg which gives you more speakers and a 600 watt 9 channel amp.. then upgrading those 4 inch speakers in all locations (front/rear/doors)

maybe the subs too...if you can get to them under the seats...

this way you don't have to bother with rewiring or cutting out larger holes into the doors and other locations and messing around with third-party amps or buidling custom boxes to house the bigger systems.

swapping out the philips to MB Quartz or Focals and using the same amp should have a significant improvement wouldn't you think? (as long s your using the upgrade sound system)

sounds like a pretty good idea...

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