not to be overlooked is the quality of the source. I too get the pleasure of comparing the sound system on a 335i (wife's car) vs. M3 (just took delivery in July) i was initially very disappointed with the low end. Couple things
1. it seemed to break in?? get better over the first month go figure
2. I replaced any song on my ipod with less than a 192kb, re-recorded at 192+ and wow 90% problem solved.
OK i tweaked the EQ settings as well, but the new settings really popped with higher quality recordings.
If you've already tweaked your EQ and still not happy, upgrade your music before taking any more radical action like replacing speakers, etc (i've heard some rumblings on other posts)
just my 2 cents
///M3 2011 e93 6MT JB/FR RG-63's HID20k PremPkg TechPkg NAV HFS +Sound