The tweeter acts as the x-over, so it's still a 4 ohm load like any other passive component setup.

 

swap out the whole system =)

 

The stock speakers are good for factory, but a good aftermarket speaker sounds better (in my opinion). I have Blaupunkts in my doors and side panels. Sound deadener will help, also play with the SSP and vehicle type settings. My car has the xB car setting, it sounds good to me, and allows more sound from the rear speakers than the tC setting.

 

with the stock HU, i added an amp to the stock pioneers up front, gave them about 40W each. they sounded pretty good for stock components, very livable if you're on a budget.

i recently changed them out for some eD 6000s and the improvement in clarity is noticable. full range stuff turned up loud really sings.

if you want to do it right get a deck with better EQ controls. treble and bass is pretty limited. however it also depends on how much of an audiophile you are. stock HU sounds plenty good for me personally. plus tbh i'd rather be focusing on driving rather than messing with my deck every five seconds, heh.