Help selecting a power cord.

So if you could only do 1 upgrade, would you do cords, or power conditioning??

Big difference.  A good power conditioner costs a few thousand dollars.  Decent power cords go from a few hundred up.  If you have the dough, I would personally buy a good power conditioner.  IMO the cheap ones do more harm than good. If you don't have the money, just buy a nice cord from a good manufacturer and try it out. 

You ask good questions. I have had some form of upgraded power cord for so long i forget which is most sensitive but I have recently been doing some upgrades. What is your equipment? You want to be investing in cords that raise the performance level enough that it would not have been better to invest in better components. Yes, power conditioner is probably the first step.

Shunyata, PSAudio, and Hydra are some to check out. These are not products to go cheap on. You should either try a couple from a dealer or get with return privileges. It should make a significant improvement. If you get a really good one, it can outlast many component upgrades.

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Then comes power cords, they are like interconnects, which cord sounds best is dependent on your equipment and tastes. It is imperative to try them. I would get at least 4 different brands... rent or borrow. Shunyata, WireWorld, DHLabs. Since you have tube equipment I am guessing you are not looking to warm up your system... that leaves out Cardas I think. Although the new Cardas Clear Beyond seems to through off the typical warm and lacking in details stereotype.
I would get two pair of each and put on preamp and amp. Typically the low power digital components would have a different type (noise reduction... and they don’t have massive current). I have used DH Labs silver as inexpensive cords as standard until I have time to evaluate. I think they are among the most cost effective. But they need lots of breakin time.

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The decision of if the power cord is worth it should be easy. The performance increase should make you say, “wow, this is a good deal!” I have said that about $1,500 power cords and $2,500 interconnects. If it doesn’t then keep looking or just wait until after your next system upgrade.

What are you using to plug-in your gear? Power strip or Conditioner?
I recommend Zavfino Majestic MKII ($324-1.5M) power cords in following order,

https://zavfinousa.com/collections/power-cables/products/the-majestic-occ-power-cable-12awg

Tube Main
Tube pre
D/A
CD transport
Turntable
Subs (last)
Roon Nucleus uses DC cable, I suggest you upgrade its stock power supply to a linear power supply.

You asked, “So if you could only do 1 upgrade, would you do cords, or power conditioning??”

Power Conditioner like Audience Forte, it comes with a very nice power cord,
https://audience-av.com/conditioners/forte-power/

If I was going to try just one pc I would choose Cullen. Those seem to make a positive difference wherever they are connected. Then move it to a different component every few days and decide where it fits best.We all gotta start somewhere.