Upgrading power cord.. proven upgrade? or crap?

like many who have responded, i didn't use to believe, now i do, but don't know how a power cord could possibly make a difference. But it does, more than speaker cables or interconnects. In my system, my CD player (Marantz SA8005) and my then preamp (NuForce MCP 18a) responded big time ti Nordost Shivas, then even more when I upgraded to Vishnus. My amps (Bel Canto ref 500Ms) showed much less improvement, but did improve a bit with either Vishnus or Acoustic Zen Tsunamis).

I cannot understand it, but here is my theory--don't think of the power cord as the last six feet of electricity coming into your system, think of your system being slammed back and forth 60 times a second as the current alternates. This is violent, with magnetic, thermal, and even kinetic ramifications.  60Hz seems fast, but in terms of sound waves of 15-20kHz, 60hz is s l o w, plenty if time to make mischief.  The poor component has to get a hold of this AC and feed it to a transformer to convert it to DC.  So anything the last six feet does to the juice will alter the task ( for better or worse) that yhe component must do.  I could be way of base here but it helps me justify the money i've spent on power cables. 😊

My first experience with aftermarket PCs was a PS Audio Lab II cable that came free when I purchased their HCA-2 power amp. It lowered noise, increased detail, and (most notably) improved bass weight, detail, and slam. I wouldn’t have said the cord was worth the $600 MSRP at the time, but it made a difference with no apparent negatives (other than cost). I used the cord on other amps, the Musical Fidelity A3cr, the Krell KAV-300iL, and noticed similar improvements. But it was not night-and-day. I eventually sold the cord for the cash, and didn’t miss it too much.

My system has come along since then. I’ve had most components for many years, and I’m highly attuned to its sound. Once I got to the $400-$500 (per meter) price range for ALL the interconnects in my system, and $1200-$1500 (per 2-2.5 meters) range for speaker cable, the difference that power cables made became about the same order of magnitude as changing interconnects. Every power cable swap at every position made a notable difference (phonostage, CD player, preamp, and power amp). I’m sure, if I could revisit the Lab II cable now, I’d think more highly of its performance.

The bad news is that power cables make a difference. But that is also the good news. They become part of the toolbox to tune your system to a fair-thee-well. For instance, using upper level AudioQuest cables in your solid-state system, and loving the detail, but finding it a bit mechanical? Add some Cardas or Audience power cables to inject more naturalness. Using (pre-Clear) Cardas cables, but finding the sound too soft? The right power cables can add more crispness and sparkle.

These are just examples - some systems work best with all cables from one manufacturer - but, however you define "high performance" (according to individual taste and listening priorities), PCs can make a meaningful contribution in high performance systems once the system reaches a sufficient level of clarity.

I see a few posters here recommending plugging your amp directly into the wall socket. Isn't that risky? How do you protect your equipment from power surges if you forego any kind of surge protection?

Completely agree with stringreen. Powercords are more noticeable than interconnect or speaker cable. They make profound differences too, but the powercords are HUGE. Try one of the dozen from Purist Audio Design. Extremely effective.

Absolute crap but if you want to waste your money go ahead. Google expectation bias before you do. 
And yes I have tried a few.