im not familiar with that card but most cards are fairly similar in their settings and control panels and such. first, i would check the output settings of both channels and make sure they are both set to the same level. On my card i can set all 8 outputs to different levels. To monitor, I use outputs one and two as a stereo pair and if channel one is all the way up and channel 2 is half way up..then i would have the same problem you are having. Next...i know you said you changed your cables...did you change EVERY cable? the ones from your sound card to the mixer as well as the ones from the mixer to the monitors? Do the LEDs on your mixer seem to light up evenly? is one channel not recieving the same signal as the other?