Upgrade Sound with a Stock look...330ci with HK

[Nelson330ci]

I need help and/or people personal experience in upgrading the sound on my 330ci. It was HK with the business CD. I want to keep the stock look, perferable keep the stock business CD deck but upgrade other components to make the sound much better.

Has anyone done this? Can you tell me what upgrades you have made and how they worked out??

Thanks.