In my experience (2008 E90 M3 and 2012 E92 M3, both with generic mufflers and custom tips), the Magnaflow mufflers drone will tone down considerably by itself after some 3000 miles or so.
Like you, at the beginning (twice
) I thought that I made a mistake because the drone is a mofo between 2500-3100rpm. But after the E90 M3 experience I had to get them again in my E92 M3. For the price ($350-$450 installed) they cannot be beat in sound, IMO.