Musical sound quality in a car is a subjective thing. That said, I'd like to offer some comments on a rear shelf speaker upgrade I recently did on my 2003 330Ci with the factory Harmon Kardon system. Like many on this forum, I think the HK system is substandard.
However, I focus my aftermarket dollars on suspension, brakes, wheels, and tires. I've never fancied the electrical modifications with major aftermarket sounds systems. It's all about personal choice.
Because of this, I thought I'd order simple a plug and play upgrade - that being the "BSW Rear Midbass Upgrade Speakers", $160 plus shipping.
The install was a snap, taking less than 45 minutes from start to finish.
When I first turned on the system I was very disappointed. All my audio settings were those from prior to the upgrade.
I experimented with the whole range of audio settings, but finally changed only the bass and fade settings from what they originally were set to.
The transformation in sound quality was significant, to my ears at least. With the BSW rear speakers I was able to increase the bass level four "clicks" (two "clicks" below max, treble set full high).
With the stock HK speakers the system would have sounded distorted and muddy. The BSW speakers seemed to provide a noticable amount of additional bass, without the distortion and muddiness of the stock HK speakers.
Also, I changed the fade three "clicks" towards a rear bias (it was originally set neutral). This was the final tweak and resulted in a really noticable improvement in sound quality.
So, just an FWIW. For $160 plus shipping it was very much worth it in my experience. Understand, even with the improvements I realized, the HK system is still rather weak compared to full aftermarket upgrades.
However, at least my BSW modified HK system is now "listenable".
Scott (aka SLO Town on E46Fanatics.com)