I installed the MB Quarts in my X5. They are on the bright side. If you like emphasized highs you will like them. I'm really not sure that the rear's tweeters need to be replaced if you go with MB Quarts. The fronts are plenty bright enough.

The other thing you will notice is how aweful Sirius sounds (if you have it). I'm back to CDs, they really sound great with the MB Quarts.