I am on X5 number two and have replaced some speakers in both.

In my 02 i replaced the midbass and the tweeters (you can probably find my pics by searching "speaker upgrade"). The tweeters made a huge difference, but the midbass dropped noticeably. I was running my same 10" sub setup, but like to have full music up front as well. The midbass speakers I used ended up frying. I think this is due to a clipped signal from turning the volume up trying to compensate for the lack of low end. The stock midbass are 2 ohm and i used 4 ohm mids. Also, the stock and crossover setting for the midbass is pretty low compared to an aftermarket component set.

In my 04 I replaced only the tweeters with some silk CDT's I had laying around. Big difference. Treble is much cleaner. I am running a sub for some bump. I do not believe there is value in replacing the midbass without going amplified. You will get cleaner midbass, but sacrifice output (IMHO).

On the other hand, replacing the midrange speaker with the BSW speaker may make a noticeable difference, I am contemplating this for my vehicle.

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2004 BMW X5 3.0i - Mine
2011 BMW 328i xDrive - Hers
2002 BMW 325xi - waiting for a new owner