Without replacing 6x9's, the best way to improve your midbass response is to deaden the doors very well. If thoroughly done, you will hear a difference. Also, without knowing the details of your set up, having a peoper xover set up is very valuable whether it be through passives or a fully active system. That will enable you to control the range of the frequencies your 6x9 are asked to handle, then you can make those speakers a bit hotter (through gain adjustment, level adjustments, or eq work) without stressing other speakers. That's why a 3-way front stage has some advantages over a 2-way front stage....the midbass speakers only play midbass frequencies say, 70hz-300hz, then the midrange will take over higher than that. You can safely bump up the output to the midbass without effecting the midrange and higher frequencies because your midbass speakers are not pulling double duty (midbass & midrange)