Geez! I have about maybe a $50k system, good ears, and have experimented with cable upgrades and other tweaks for years. Here’s my take:
If you have ample, good, clean power feeding your components upgrading the power cables makes essentially no difference. If you have to listen really really hard, until you think you’re hearing a difference-- you are, but it’s in your head, not the cable-- so long as your cables are not damaged or set wrong.
Interconnects will make some small but occasionally important differences-- however, this will not equate to spending more money equals better fidelity. Beyond a certain level of build and materials they don’t improve per se, you cannot rank their sound quality by price, you choose what you think sounds best -- but remember confirmation bias will often induce you to believe that paying more for a thing means it must be better, whether or not the difference is real or in your head.
Contact enhancers -- these can help if your connections are poor, oxidized, dirty, etc. This is what I think some people hear after spending a lot for some contact enhancer tweak-- what they are really hearing is that they have cleaned up and reset all of their connections-- and likely NOT the fluid they have just applied. Same thing with fuses. Beyond that it’s almost certainly placebo.
Designer fuses -- among the biggest scams out there IMO -- but sometimes replacing an old fuse set in a heavily oxidized holder-- with a new (and correct) fuse-- will make a bit of difference-- but more because the contacts have been reset just by changing the fuse, NOT because that fuse you spent $100 or more on has some special audio magic.
Why do I say this? Experience, and, that (for decades now) when A-B testing is done with these sorts of items on a good sounding, well connected system, no one is able to quantify the cost of the tweak with the quality of the audio. No one.
So look, if you have the money and prefer an interconnect cable that costs thousands of dollars per meter, then why not?
But many people have a decent system and not a lot of money. They’re looking for upgrades that they can do to make it sound better, but really don’t have the money to do major equipment upgrades. They might have to put that mega-expensive power cable on a charge card and pay a ton of interest on it before it’s paid off for example.
Some may not have been in the hobby long enough to learn some of these lessons and are attracted by the "amazing claims" that many but not all of these sorts of companies are prone to make. I do feel like this is kind of taking advantage of people.
If you have decent components and cables, and can’t or don’t want to replace some or all of them, but also feel that your system is lacking that certain something, as boring as this sounds, look to the room, not the gear.
Start by getting your speaker placement right, try some minimal room sound treatment to absorb reflections, clean up your wiring behind the system to avoid interconnects and power cables running parallel to each other, and then live with those changes for a while. You will almost certainly hear a profound and obvious improvement in sound quality that goes far far beyond what you will ever get out of a bottle of quantum contact cleaner, stupid expensive power cables, or interconnects that site some completely goof-ball physics theories that have informed their design and construction.
More than anything else-- keep your mind open, live with the changes for a while, and don’t try really hard to hear a difference. If there is any improvement you will come to that conclusion naturally over time as your ears acclimate to the changes that you have made. Good? Worse? Nothing? Only time will tell.
Now go play some music!
