I now have Steathboxes in the back, a Kicker amp and ADS in the front.
The speaker made an unbelievable difference.
Note: The stock head unit blows for FM sound, but is quite good for CD sound.
You can crank the music to the point were you can't hear anything (and probably beyond) including the engine and the sound is perfectly clear.
I did not get a higher end version of the ADS due to the high background noise in the M3.
FYI: The stock speakers are unbelieveable crap. As upgrading the stock head unit makes a night and day difference for FM sound, a change of speakers makes just as big a difference in sound all around.