Where do cables upgrades have the most impact?

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.

Speaking of brands and models, you will want to try what you never or almost never see used for sale yet what sells new in significant quantity. I am talking real cables and cords not $200 entry level junk.
How would you know ? Not sure, find a way.

It also depends on wall current.