You don't understand, do you?
Using the DAC volume control at any volume setting but max will decrease the S/N ratio from the point of the OPA1602, making it more susceptible to the "dirty" external PS. Using DAC built-in volume control is never a "Hi-Fi" circuit design. You can ask people at the Dedicated Source Components forum.
DAC built-in volume control is NOT equal to the implementation of using a dedicated digital volume control chip at later (pre-amp) section of the circuit.
Using external AMP, I can max out the DAC volume to maximize S/N, making it much less susceptible to "dirty" PS.
I suppose you do use shielded coaxle or twisted pair interconnect cables, don't you? Why don't you use straight bare wire interconnects, and say it is the dirty electromagnetic environment which causes the noise?