Your speakers may be "out of phase", meaning that the positive and negative connections are reversed on at least one of the four speakers. All speakers must work in harmony, meaning the cones must move in and out in sync. If not, they will try to cancel out each other, leaving missing bass, no imaging, and other phase anomilies that will give you headaches. An easy way to test is to use the balance and fadar to compare sound with one speaker compared to all speakers. If they are out of phase, it will have more bass and in general sound better with one. Use a D-cell battery at the head unit speaker connections (unplugged from the unit). Figure out which is supposed to pos and neg, touch them to the respective battery terminal, the cone should move outward from the speaker frame when touched. If you find that one moves inward, switch those connections and you should have better sound.