BASS BLOCKS MIGHT HELP(FAIRLY CHEAP TO BUY SO MIGHT WANT TO JUST GIVE IT A TRY), BUT THE PROBLEM IS PROBABLE CAUSED BY THE FACT THAT YOU ARE TRYING TO PULL POWER FROM A HEAD UNIT THAT CAN'T PROVIDE IT. IF YOU ARE STILL USING THE FACTORY HU IN YOUR VEHICLE, THEN I WOULD START BY REPLACING THAT. IF YOU HAVE REPLACED THE HU, THEN I WOULD LOOK AT YOUR SPEAKERS. IF THEY ARE STOCK I WOULD REPLACE THEM. IF THE HU AND SPEAKERS HAVE ALL BEEN REPLACED ALREADY, THE ONLY THING I WOULD RECOMMEND TO DO WOULD BE TO GET A SMALL AMPLIFIER TO RUN THE 4*6(PROBABLY YOUR BEST OPTION). THIS WILL CLEAR THINGS UP BECAUSE THE AMP WILL PRODUCE A CLEARER MORE POWERFUL SIGNAL.
iF YOU REALLY WANT TO GET GOOD MIDS AND HIGHS AND HAVE SOME MONEY TO SPEND, TRY REPLACING YOUR 4*6 WITH A SET OF SEPERATES(SPEAKERS WITH SEPERATE TWEETER, MIDRANGE WOOFER, AND CROSSOVER) AND AN AMPLIFIER. IT MAY REQUIRE SOME FABRICATION ON YOUR PART TO GET THE SPEAKERS TO FIT, BUT AM FAIRLY CONFIDENT THAT THE RESULTS WILL BE WELL WORTH IT.