Improving BMW E39 audio.

Anyone with any specific suggestions for improving the OE system in an E39?
Car has the 'Business HU' with cassette, and 6CD changer in the boot.
System has plenty of speakers, but IMO the OEM sound quality is poor.
Both treble and base seem weak, without any clarity or 'presence'
I'm not looking for a very good u/g, but if a reasonable improvement can be
made by simply replacing the main speakers front and rear with some better
spec ones, I reckon my limited knowlege of car audio could cope with that.
:-)
Any suggestions?
TIA.
Mike.