FYI: Most DC-DC PSUs don't have separate grounds; they usually share a common ground with the 12V input. Even if your PSU had its own floating ground, the PC's soundcard uses the same ground as the motherboard and all the peripheral cards, etc, so digital noise can still find its way to the low-level outputs.

Much of the "digital noise" heard is a result of poor shielding and board design methods used by the manufacturer of the sound card. I have at home a sound card costing several thousand dollars made by Antex Electronics that has an aluminum enclosure around both sides of the DAC and output stages. This card outputs studio-grade quality from a standard PC.

The only way to completely remove all digital noise would be to have separate analog and digital grounds, which PCs do not.

The best way to get low noise with common hardware is to use a good sound card and make sure things are well shielded and quality cabling is used.