I currently have a base 328i (well w/ Tech package anyway) as a loaner while my car is @ the dealer. The audio system is OK - not horrible, but then again I'm listening to Apple Lossless files and this car has the enhanced USB, so I'm not stuck having to deal w/ the iPod's crap DAC, like in my car. But my point is that b/c the source is better, the SQ is better.
Things to think about:
- Crap source (mp3, m4a, sat radio, BMW Y-cable) = Crap output
- Simply changing the speakers by themselves isn't the cure-all; I'm not sure what kind of amp the base system uses (if it uses one at all), but a new amp + speakers probably the most effective upgrade ... or hell, maybe just a new amp.
In my case, I'm having the combox retrofitted, which should solve my issues. Otherwise, the HK system is quite good. Of course, I'll agree that I too enjoy tight effective bass. The HK does it OK, but no comparison to the 12" sub in my previous Mazda3.