Which power cord should you buy first

+1 for Calvinandhobbes.

cable upgrades on my DAC and Power amps provided different forms of improvement.

my DAC needed filtering to remove harshness and clean up the sound. My amps needed a more open current flow.  My power cables did not work well in my DAC and my DAC cable did not work in my power amps.

if I had to do one first think I would do the DAC cable because that ‘took away bad’ whereas the power cables added good, if that makes any sense.

But that’s in my system, with my electric, etc.

I don’t think I included cables in my profile so for reference:

I have a dedicated circuit for my system.

power cables for monoblocks and preamp are custom cables from Triode Labs/finale audio (maker of my amps)

power cables for DAC are Anticables level 3

If you can tell the difference between switching power cords you are truly a remarkable human being.

I upgraded my power cord cause I feel better when I spend money on equipment. It makes me happy to spend money and that’s why I do it.

I have a Power isolator from transparent. That made me feel even better to spend more money and I think this one probably was helpful.

There have been tests done with no big increases in power or sound ...I just used the power cable that came with my amp...

@emergingsoul +1 on "If you can tell the difference between switching power cords you are truly a remarkable human being.” comment

I replace original high end stock cords if I need more flexibility, length adj. etc. Maintenance such as cleaning contacts. tightening screws etc also helps. To keep originality of devices I also buy original manufacturer cable replacement, which always “sounds better” than aged one! 

I've heard both approaches (sources to amp or amp first) so probably no wrong answers. Ultimately you need to get them all done to get the best from your system.

That said, I'd start with the Amp. That way you'll hear each improvement as you go back in the system to preamp then source PC's and interconnects. 

The Shunyata NR PCs are great. 

Have fun!