I've owned a dac 1 USB for a couple of years, and just took delivery of an HDR. Although I haven't compared them side to side, I have had them in the same system and have not noticed a difference between the two. On the other hand, the two systems couldn't be more divergent. Tannoy with vintage tubes vs Soundlab with high power solid state. Have you tried different power cords/conditioners, as they are sensitive to them?
Benchmark DAC-1 vs DAC-1 HDR
The Benchmark DAC-1 has recieved stellar comments on this site. Their HDR offers additional benefits and appears to have no drawbacks in comparison (other than price). I know they switched op amps between the DAC-1 and the DAC-1 PRE and DAC-1 HDR. Has anyone found a downside between the models as one goes up the line ?