if you want to reuse stock locations,my understanding is that not many speakers fit into bmw's.that's why i upgraded my hi-fi system with the bsw speakers-
http://www.bavariansoundwerks.com/
this kit replaced my 8 speakers,left the underseat subs as is.i'll upgrade those later.they are tailored to fit in the bmw's spaces and keep an oem appearance,with no new wiring.they say they will eventually make an amp,but it's been a long time in development.there are certainly higher end speakers out there,but these are very convenient,and are a big improvement over the stock hifi speakers.they appear of high quality,and my bmw mechanic friend was able to install them in about 20 minutes