The problem with stock stereos are not the speakers...it's the lack of power coming from your head unit. Speaker distortion is due to lack of power.
Two suggestions:
1. BUy a new head unit. Most head units are putting out about twice the power as most stock head units unless your stock is a premium sound system. By adding the new head unit, you will get more power to your speakers which will IMMEDIATLY make them sound better. ($200 + $50 install)
2. Keep stock head unit and add after market amp. EIther a 4 channel amp to all 4 door speakers or a 5 channel amp to all 4 door speakers and then add a sub later on. THIS IS THE BETTER OPTION of the two since you will be increasing your power by about 4X from the original stock stereo. ($400 amp + $120 wires + $75 install)
---Then if you feel the need to add subwoofer ($200) get a 10 or 12 inch single sub in a sealed box. This will compelte the system.
---Also, upgrade door speakers at some point. Start with fronts (only add component speakers) and then move to the back ($100 or less for rears).
