Where do cables upgrades have the most impact?

Speaker cables wuld be my recommendation. Make sure that size and length match the amplifiers electrical performance. When this is done it ie easier to listen between the different interconnect cables. Best Mik

The shorter the better? When talking of cables, this statement is often presented as an axiom, although after several experiments it is not even a theorem for me. Using the example of intercon cables (the same manufacturer and the same model), I can say that in most cases a cable 1 or 0.8 meters long sounded better than half a meter long. I suspect that the matter is in some "wave" nuances, but I would be happy to know the expert's opinion.

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

For me personally I do not filter my power. I never heard a serial filter or / power conditioner to my liking but I have not heard them all.  What I have heard sounded to cleaned up for me. Borring is what came to mind.    

I do have a separate HiFi power group. Does this lead to a better experience ? I don't know. Theoretical it could. 

A power strip in a wall-outlet when you have to connect more than one or two devices is a logical option. 

My power section is build around a good 8 piece power strip that goes in the separate group wall outlet.  From there I have plugged in my devices in order Power AMP and the rest. When I use a non seperate wall outlet I think I cannot pick it out in a AB or blind test.   

In a powerstrip digital devices are adviced to be last in line. I am not able to hear differences in other combinations. I just followed this advice because it cost me nothing and the plausible explanation came from an experienced person.    

I have power cables that are 6 feet minumim lenght. Read many sience, theories and opinions about the minimal power cable lenght.  If the theorie and science is wrong than we waist a lot of material. If it is correct then I have the right cable lenght and created piece of mind in this section.  

Still changing the power-cord on an Pre-Amp / AMP or Digital device like a Dac is a good place to start. 

Start with the basics. Dont over do it and go from there.  

Agree with above about speaker cables, different speakers have their own kind of sounds that tend to work better with particular types of cables.  Once you’ve optimised the speaker end of things then you can isolate and play with different parts of the chain.  At the end of the day wrong speaker setup will give you wrong sound.

Info from audio shops indicated order of importance #1 power cord, #2 interconnect followed by speaker cable