You could also try Herbie's grungebuster + PTFE washer pairs to isolate the PCBs in your DAC as I have done to good effect. You have to unscrew the PCBs to put them in but this needn't be complicated depending on how your DAC is laid out.
Also, the Synergistic Research ECTs are very helpful for reducing high frequency noise in digital equipment - such as in DACs, USB converters and also in power supplies/conditioners/etc. But for the ECTs to work at their best you may also need to have the SR FEQs in the room. I'm not sure about this last point, but I do have the FEQs running all the time, and I can say that the ECTs do something for digital components and power supplies that other tweaks don't attempt and they do work.
Also, the Synergistic Research ECTs are very helpful for reducing high frequency noise in digital equipment - such as in DACs, USB converters and also in power supplies/conditioners/etc. But for the ECTs to work at their best you may also need to have the SR FEQs in the room. I'm not sure about this last point, but I do have the FEQs running all the time, and I can say that the ECTs do something for digital components and power supplies that other tweaks don't attempt and they do work.