Echoing what @mclinnguy said, If you have a power conditioner, I would get a power cable for that. My thinking is that results in cleaner power to all downstream components resulting in less effect of placing a power cable on those other components.
If there is no power conditioner in the picture, I think mainstream advice might be to put the power cable on the amplifier. But, I might suggest a power cable for a DAC. I have found a surprisingly large beneficial effect in clarity and resolution with the right power cable on my Denafrips Pontus II DAC.
However, I will say that especially on my DAC, finding the right power cable was very important. I tested a whole bunch of power cables on my DAC. 6 sounded worse than a basic $15 Tripp-Lite Heavy Duty power cable from Home Depot, 1 sounded about the same in overall sound quality (though it still sounded different), and 3 sounded better than the Tripp-Lite cable.
Too much noise filtering technology (or the wrong filtering technology) in a cable seemed to result in a dull and muted sound. A generic $15 power cable actually sounded better than a whole number of cables costing between $500-700. A $95 power cable ended up sounding best.