Obviously, if u dun dare (or plain dont want) to change the HU (warranty, looks, it's your FAMA's car, etc...), then you can continue to DREAM of better quality music.

You can do the following and get "marginally better" sound quality, but you will never hear the max potential with stock HU:

-Change front components
-Add woofer amp and woofer
-add components amp
-continue with life, as any more is no improvement with stock HU.

First and foremost Upgrade your HU (Head Unit).

As they say: "Crap in, crap out".

And nothing says "Crap" more than than ANY stock HU.
Speakers, amps, woofer, bla bla bla, nothing will change the sound more than your player.
Then next only change component speakers. The internal amp of your (hopefully already upgraded) HU wont be able to unleash the full potential of ANY component speakers, but it will still sound much much better.

Then can add "amplifier for subwoofer" and the subwoofer with it's enclosure, to re-claim the lost bass frequency that component speakers cant reproduce.
A good HU will have ALL the necessary digital controls and outputs ready to integrate a sub no prob. Gone are the days of separate pre-amps, crossovers, and EQs.

By now, you would want more power (louder with no distortion) from them front components. Time to add in another amplifier for your front components to unleash max potential.

Obviously the Subwoofer amp and front-stage amp can be a single unit 4-way amp, or 2 units of 2-way amps. I recommend separate units. Brand and model up to your budget.
That would be the basic upgrade path for better sound for ANY car.
Once the above all done, and you still serious, ask more and we'll tell more =D
The above alone will cost you at least 1 month salary.

Happy ICEing =D

Practically 95% of stock car audio sucks.

The ones that are decent (from BnO, Dynaudio, Bose), you will only get from cars with street price RM150K and above.

Hence:

Your car price >150k, maybe.
Your car price <150k, all suck.

how much u are willing to spend on the front speaker?