All guidance seems to point away from using a sound card. I recommend reading some of Audioengr's posts on this. An external DAC will make a big difference, as well as the way you get the signal into the DAC. Not all USB devices are created equal and some digital connections work better than others.
You've obviously done a lot in this area already. I recommend trying a better DAC (for what it's worth, I wasn't impressed with the Cosecant or Benchmark USB DAC-1, but others here seem to like them). And also connecting to the DAC using an Empirical Audio interface, such as the Turbo-2 or PaceCar. This should outperform your external sound card solution, but the cost may be much higher.