This is a question for all the audio experts.

I've always been content with my non-ML system. I know it's not fantastic, but I never felt a need for more... that was, until I took a ride in my friend's Mercedes. It had the harmon kardon Logic7 system, and I was very impressed. So it got me thinking, if I just replace all the stock speakers, how much of an improvement can I achieve? Or is there an even easier way of getting better sound? I want better overall sound, and I like my bass so I still want it to hit hard, but I don't want to install subs.

Thanks in advance.