I own two expensive high-end power cables and two standard cables.
The standard cables are the ones I received from Naim Audio when I purchased my devices.
The devices I own are: NDX2, NAC 282 (connected to NAPSC2), NAP 250 DR, and HiCap, which is connected to the NDX2 and NAP 250 DR according to Naim’s official recommendation diagram.
Based on your experience and testing, which devices should I connect the expensive high-end power cables in order to achieve the best sound quality?"
There are so many kilometers of wire from e.g. a power station to substations and after that to my house, that I don’t believe the last 1,5 meter in this chain will make a noticable difference
Have you evaluated it? Doesn’t sound like it to me 
Because I could make a demo for you with only one cable changing and you would easily hear the difference. 100%.
Absolutely. Put a microwave white cheap cable on each box, you want notice any difference. Or it’s only on your mind.
When I hear you say you don’t believe it, I can immediately conclude you haven’t even tried it. Therefore, I’m sorry, but your vote doesn’t count. 
You have the cables, so I would recommend you try them out in different combinations. It’ll be an eduction of a sort, and maybe even a bit of fun. It’s probably best to start out with all standard mains leads and then introduce the “expensive high end” cables one by one, trying on source, then pre, then power, and even on the NAPSC (you might be surprised).
Or even put the expensive cable on the microwave and see if food tastes better.
…or try the expensive cables all the way back to the power station.
I just updated the standard cable to 1.5mm 2 so that it complied with safety regs to have the must use 13A mains fuse. 
Safety and regs mean more to me than fads.
Based on ChatGPT:
Question 1
I own two expensive high-end power cables and two standard cables. The standard cables are the ones I received from Naim Audio when I purchased my devices.
The devices I own are: NDX2 Sreamer, NAC 282 Preamplifier, (connected to NAPSC2 small power supply), NAP 250 DR power amplifier, and HiCap power supply, which is connected to the NDX2 streamer and NAP250 DR power amplifier according to Naim’s official recommendation diagram. Based on user experiences and testing, which devices should I connect the expensive high-end power cables in order to achieve the best sound quality?
Answer 1
Based on Naim’s ofÏcial recommendations and user experiences, the optimal allocation of your high-end power cables is as follows:
Question 2
OK understood. The first expensive high-end power cable to the NDX2 Streamer.
What about the second expensive high-end power cable priority connection?
Answer 2
Based on user experiences and Naim’s design, the NAPSC2 power supply controls circuits of the NAC 282 preamplifier, handling non-signal path components. The HiCap power supply provides a dedicated power supply to the audio circuitry, directly influencing sound quality. Therefore, connecting your high-end power cable to the HiCap power supply is likely to yield the most noticeable improvement in audio performance.
Summary
This configuration aligns with Naim’s guidelines and user experiences, aiming to maximize your system’s performance by providing high-quality power to the most sensitive components.
It’s always nice to see people given a good reason to sneer.
Fortunately, @Richard.Dane has already suggested what I would encourage - make decisions by listening (perhaps involving a couple of friends and some blind listening), rather than believing theories (however robust) or views here.
Of course, if focused listening shows that you can’t tell the difference, many here will be unsurprised.
Your profile shows No hifi system.
But probably powerlines on fridge, microwave, washing machines….
You would require 4 expensive high-end power cables to make this a valid effort.
In my experience, it’s difficult to make your system sound better than stock cables , but it’s sure easy to make it sound worse .
This might be an unpopular opinion, but how much could you sell the two high end power cables for? And instead add a PSU to NDX2.
In our experience Powerlines add a little something but they won’t add as much as a power supply will to your NDX2.
Stock cables are mediocre. My experience is that it’s actually easy to improve on them.
Naim’s stock cables are no afterthought - they were carefully selected after listening to many different alternatives. My favourites are the ones that used Pirelli cable (from just down the road IIRC).
Expensive aftermarket cables won’t necessarily be an upgrade, and sometimes are quite the opposite. I remember once being asked by a dealer to listen to a particular mains cable on the Naim system he had set up in the shop. It was an interesting looking thing, like a brightly coloured snake that had swallowed a rat or a rabbit and created a big fat lump in the middle. He told me the retail price, and I thought, well, for that money it had better be something amazing. He pressed play on the CD player and played a track I knew well, and it sounded pretty good. Then he fiddled around behind the system and played the track again… OMG! It was like everything had gone out of phase. It was bizarre, and really rather awful. I suggested he get the rep in and do the same dem for him, then ask him to find out what had gone wrong. I never heard what happened. I’m still somewhat intrigued…
Depends if you want to reshape the sound away from the original design.
Bad news for the cable doubters, but I recently added Shunyata PC’s to my NDX 2, SN3, & HiCap DR (sequentially). The improvement from the included cables was so easy to immediately discern, I doubt the honesty and integrity of any person who heard pre- and post- power cable upgrade and still says “those don’t matter”. Having bought smartly on the used market, I have spent less than 1/4th what I paid for the HiCap DR alone as a preferred customer at my HiFi Dealer (read: less than msrp).
Across all cables, the magnitude of upgrading the 3 PC’s was roughly equivalent to adding the HCDR to my SN3 (which was a clear and obvious upgrade).
Granted this is very cable specific and some can hurt more than they help…regardless of price. System synergy matters.
And this is with the open acknowledgement that Naim’s stock power cables are perhaps some of the best stock power cables in the industry.
My local power station has closed recently so not an option, unfortunately, even if it were possible. It was top class electricity. I know because I used to make it. 