I second Steve's suggestion. Even though the DAC supports async USB, it's worth trying out a top-notch USB/SPDIF convertor (on trial?) to see if it betters quality. If not, then the investment is worth going towards a DAC upgrade. In my experience power conditioning is a must on your source. Even a Tripp Lite isolation transformer makes a difference.
Steve, can you recommend any signal transformer to go between the DAC/amp?
Steve, can you recommend any signal transformer to go between the DAC/amp?