Where do cables upgrades have the most impact?

@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.

Personally, I've found that the ICs have been the most profound improvement in my system.  I think the speaker cables would be right there also, but, most systems have SCs that are similarly priced as the PCs and ICs.  To get an equivalent jump in performance, the generally much longer SCs will need about a double the price difference to achieve the same gains.  The Cerious Technology Matrix SCs would be that type of jump for me.  The Teo ICs used as doubles to each component from source to preamp is an extremely big improvement.  I've kept the same sets of PCs for many years while I've changed the other two a few times.  

Big jump in performance of the ICs when I went to all silver cables from either Darwin or Amadi after having several Morrows, DNM, Audioquest, The change to Teo GCs brought a more organic sound that was also a real game changer in my system.  

Using a pair of Core Power Equi=Core 150's as my PCs in both the TRL DUDE preamp and as the power coming from all my sources was a very good improvement in dynamics, clarity and lower noise floor.  Other PCs have been good such as the Less Loss Original and Wywires Juice II's.
I probably will have to jump up in price to significantly beat the PCs I currently have in the system.

My speakers are VMPS RM40 BCSE MLS with most of the upgrades or the Vapor Audio Joule Black.  The amps are Nuforce Ref 9 V3 Se monos with the level 3 TDSS upgrades.  The VMPS are very detailed and transparent where the Vapor's are just a little more sophisticated and clean sounding.  The amps are just freakin' awesome with all the speakers I've tried them with.