Speakers matter most.
In general.
Especially being 10 years old, I'd expect that new speakers would help most for "muddy" sound.
Crutchfield has good tools for seeing what speakers fit, customer reviews, etc. If you buy there, they supply good interface kits and instructions.
http://www.crutchfield.com
My looking around there for a 2002 S80 (assuming non-surround sound) lists these (at $100/pair) as being highly rated - http://www.crutchfie...b651s.html?tp=105
That said, if you like your music really loud, you might need to add an amplifier as well. I haven't done that myself (never had the need).
HTH a little.
Cheers,
Scott.
