If you're using USB, your source is a computer, and quality here depends in a big way on the USB implementation. Poor USB circuits at the DAC, when fed from a laptop, will get beaten by coax coming out of a decent CD transport. In my experience, most USB DACs under $1000 are limited to 16/48 over USB, and have poor implementation. To get really good sound, you need a decent USB/SPDIF converter, in most cases, unless you're spending over $1000 on the DAC. Suggestions on a DAC-only USB solution? Benchmark DAC-1 USB and Wyred for Sound DAC1 or 2 is where I would start.
I bought a PS Audio DLIII, and the laptop-DAC path via USB could not compete with my Denon changer as a transport. I bought a Vlink and the Jplay plug-in, now the sound is a little forward, but I have a wider soundstage, better depth, more detail. Planning on upgrading the converter.