Just stick some capacitors in, non-polarized 200 microfarad-which is rated at 100volts DC, if you dont know what this is, ask at a maplins or wherever you get your components from.
These generally stop the lower frequencies coming (i.e-the bass). One cap for each speaker.
These should be connected from positive wire coming from the HU to the positive end of the speaker, (dont matter which way around the capacitor is).
This is a quick fix, but should help the standard speakers out (by not distorting)- This is only if you dont want to replace them.
Hope it helps.