Where do cables upgrades have the most impact?

There is nothing to ponder it's basic marketing. 

@teo_audio  

To try to at least build or modify a single piece of gear. Or that built piece of gear, being modified at least once. In some way. To get an understanding of the complexity of the signal in the boxes. This might be a way.

Point noted. I will move in that direction, holistically.

Both of your posts have been helpful. Thank you!

@almarg  Thanks for your post. All of your 'factors' are important in one's consideration, as I myself recently learned, via you. : )

1extreme,
" Curious if those saying the power cable had the most impact are plugging into a power conditioner or a wall outlet?"

Both

Yes, there is a consensus - in most cases speaker cables are least important. And that's exactly what I would start with. After that or at the same time with that - power cord on the power amp. This would complete power amp/speaker cables/speakers unit. If you use power conditioner/regenerator for the amps I would first put power cord there.
In my limited experience the biggest impacts were the RCA interconnects from the source, either analog or digital, and power cord from the wall. My Nottingham tonearm wiring goes uninterrupted from cartridge to RCAs and I have no plans to have it rewired, but it can be done.