Temporarily turn off your subs and listen to your 6x8s alone, If moving the balance to either the left or right results in more bass, then they are out of phase. If they just sound anemic and underpowered, that may indeed be the issue, just that they are not getting enough power. what kind of deck power are you running to them? Facory deck or aftermarket? Are they mounted in doors, dash, or back of the your car? Mounted to plastic, metal, or wood? There are lots of factors that will affect the amount of bass you will hear from a speaker, some testing will help to determine if it is power, wiring, mounting, or just a poor speaker.