"That would be the DAC, as opposed to the transport"
Not necessarily -- in a full DCS stack the best place for the first power cord upgrade is on the clock ...
The obvious answer is where the influence of greater power stability will be most impactful but that also depends on how the power supplies themselves are designed.
Unfortunately you probably have to try but as a rule closer to source (i.e. in the original question that would be the transport actually) as any problems created there cannot be corrected downstream