First off, be prepared to open your wallet... Any upgrades you decide will cost. Probably the cheapest and best upgrade you can do will be the BSW speaker upgrade, or JL Audio sub, or both. The speaker upgrade will be something u can easily do on your own, but the sub you may want to consider having someone else do since you have to install a amp to power the sub. It can get complicated depending on your experience. Replacing the DSP amp is a real challenge and if you decide to go this route you may be best replacing the entire system, the route I went, but still you are faced with the same challenges as the factory system.

The factory system in my opinion has the clarity which should please most, low end is lacking but not the worst. Here is where I see improvement could be made, so depending on what you are searching for in a sound system you can add the sub or change speakers or both. If that is not good enough then consider a complete system change and you would probably want a larger sub system to give you the sound you are looking for...

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Robert

1994 740iL
2001 X5 4.4i Sport
2004 Kawasaki ZX-6R
2009 VW Jetta S