I have the BMW 328I 1996 with stock headunit/6CDchanger. So far the only upgrade done is replace the rear speakers (4" with new 4"). Sounds better but not much!
What is the next best upgrade:
1. Replace headunit (but what headunit works with factory CD changer??) $200+
2. Replace OEM amp in trunk with OEM Harman Kardon AMP? $75
3. Replace OEM amp in trunk with aftermarket 6 channel amp? $200+