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When you say that the audio system is not what you expected what do you exactly mean? The X5 audio stock system is not bad but definitely lacks good bass. If it is only the lack of bass which bothers you then you can easily upgrade it by adding a subwoofer and a sub amplifier. Adding a subwoofer and amp for the subwoofer is what most people need. Here is an article which explains how do to that. http://www.xoutpost.com/showarticle-237.html

Now if you want a more drastic upgrade then you cannot just replace the speakers without replacing the stock amp. The X5 stock amp uses built-in crossovers. That means that the outputs of the stock amp have filtered frequency range and that the stock amp and stock speakers are matched to each other. Unfortunatly that also means that if you take the output of the stock amp to aftermarket speakers or to an after market amp for that matter you will never have a good signal. Therefore is you want to upgrade your system you MUST replace the amp too.