1) Run dedicated power wires for your Opus and amplifier. Use 4 guage and a fuse at the battery run to a distribution/fuse block. Use 8G from your block (with fuse) to your amp. Use 10G from your block (with fuse) to your Opus. Run 10G ground wire from your Opus to a good ground point (bare metal bolted down). Run 8G from your Amp to the same ground point.
2) Keep your RCA lines as far away from your powre lines as possible (also far away from ANY power lines in the car) Ideally they shouldn't cross the power lines either.
3) Make your RCAs as short as possible. Since your using a USB sound card it should be adjacent to your amp so you can use RCAs that are only a few inches. If you do this step alone and you still have noise, it's almost certainly coming from your power rails directly into your amp. Short your amplifier inputs and slowly turn up the gain. If you hear any noise then it's from your power rail and/or the amp is busted and/or lousy.