I would first pull all the RCAs, they verify you have the input setting at 4 channels and not summed. Then take the two channels that drive the 6x9s and plug them in. Then listen for the difference - whatever channels seems softer swap out with the opposite channel without changing any settings and see if the lack of volume follows the RCA lead or it isolated to the amp. This will tell you which way to go... (towards the speaker or towards the headunit/RCA cables)