Which power cord should you buy first

It pays to experiment, but with my specific gear, power cable was better on the source first. 

I realize this is an old topic.  It's good getting current experience.   Appreciate everyone's input .  My Pass labs 350.8 amp comes with a pretty beefy power cord with its own direct power line.  I have a separate Lumin U2 streamer and La scala DAC.  Sounds like experimentation is in order between these.  Thanks again 

@salc trying is the only way to find out. Everything matters.
However based on my experience, I follow a simple rule as I proved over and over it works every time - start from speakers and go up the chain - speaker cables, amp power cord, interconnects into amp, etc.

Plug the amp directly into the wall outlet without power conditioner

First power cord upgrade is for the amp - 10awg minimum, best possible cord you can afford

Second is for the preamp. Then on power conditioner feeding source components, then up the chain - preamp, DAC, streamer/transport

Also, I owned 3 Pass Labs amps - X250.5, XA30.8 and X260.8 monos. The generic 14awg stock power cord included with the amp is enough to get you up and running. But…every time a good power cord made a difference on these amps in my system. Even the affordable Audience Forte F3 beats the stock cord.

I have also owned many Pass, or Nelson Pass designed, amplifiers going back to the mid 1980's- and have found some improvements in replacing the beloved supplied power cord on some of them. I am currently using Audio Sensibility Statement cords on my XS150 monos - and was happy enough to go with them throughout the rest of my system. I will say - what I think made the most difference - was plugging them directly into their own dedicated 20A circuit. 

I’ve lost count as to how many power cords I’ve tried. My modus operandi is to buy a pc for an intended component. Evaluate it there for two weeks or so. Then reinstall the previous pc - and try the new one on other components for a least a few days each. Finally putting it where it sounds best for my ears. After cable swapping for 20+ years, I’m always surprised where it ends up. Even last week, I experimented with swapping pc’s that are already in my system. The results were very educational.

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. . . comes with a pretty beefy power cord

In my experience, beefy doesn’t correlate to SQ. There are many other factors involved.