Welcome! I bought an 'older' 7-series and started upgrading things here & there. We all find out sooner or later you need good tunes to match your ride. First step for tunes is you are going to need a an external subwoofer; the smallish speakers that fit in the tight spaces of the doors/shelf just aren't sized to move enough air. The problem to get balanced sound in BMWs (balanced = balanced, not Bazooka-like weaponry) is almost strictly in the domain of the low-end. Just go to eBay and search on BassLink. Not only will you find BassLink subwoofers under $200, you will also find BMW-specific install kits. The $250 or so you spend on the upgrade will likely become the most memorable $250 you will *ever* spend on your Bimmer.
You can read a post/article at:
http://www.wakajawaka.org/bmw-audio-upgrade/
or just go to eBay and do an eBay search on BassLink.