Which power cord should you buy first

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!

@salc - as calvin and others have said, power conditioner first. But then followed by server/streamer, then pre, then dac and last, amplifier - seemingly counterintuitive, I know, but it’s all about the source, as evidenced by the biggest changes to sound realism. The dac isn’t part of that source, it comes after the server/streamer, and the pre is the first point of amplification, which is a source of sorts. The dac comes after the pre, only because it sits before the amplifier. I have never heard sound quality benefits from good cables to amplifier the way I have from the server, pre, and dac, in that order. And if your server/streamer has digital preamplification, then all the more reason  : ) - just a different take on a long debated issue.

In friendship - kevin

Which of your components is going to inject the most electrical noise into your power supply? Most likely culprit is a class D amplifier, followed by a computer! Least likely might be a turntable, provided it does not have a switching power supply.