Hearing is believing?........power cables.......

@decibell

Explain. Why yes it is quite simple. Power can be noisy and badly designed components and appliances can make the power in your home noisier (even to the point of affecting other components) Switched Mode Power Supplies are a major culprit as they draw high current loads and work at high frequencies.

Think of a conventional power supply like a dam across a river - it stores the river energy and allows the river below the dam to be controlled and remain independent of the flow above the damn. A well designed power supply would be like a large dam across the river. The flow downstream is perfectly controlled and smooth. A badly designed power supply is like a very small dam or none at all and changes in the river flow up stream are transmitted down stream.

An audio device with a well designed (usually massive) power supply sees only consistent smooth filtered power to the audio signal components and is immune to what is going on in your household electrical power. A power cord does NOTHING for a well designed power supply. You only need an adequate rated power cord and you are hearing the music as good as it gets.

An audio device with a poorly designed power supply will see all forms of household power fluctuation right at the audio signal components and will therefore sound distorted and noisy at various times depending how clean the power happens to be. A different power cord may indeedhelp a badly designed audio device as most of the Fluctuations in home power are reaching the audio signal components.

So very simple. Badly designed power supplies allow household power noise to reach those components directly producing your audio (line level signal etc) and very well designed audio equipment with large power supplies (good transformers, large caps and proper separation of power from line level) relative to the audio signal components will NOT.

I'm also of the opinion that, when upgrading cords, most of us go with a fatter cable than what came with the unit, so the increased conductor size increases both the current available (assuming there's not a bottleneck somewhere else, such as a conditioner or distribution box with smaller conductors) and the grounding and neutral path available for noise to exit the component. This is in addition to any smoothing or filtering aspects a particular PC may possess.

@shadorne 
I appreciate your perspective - which I understand and agree to when these circumstances apply. In my case though, I don't think they do. Each piece of gear that I use has it's own dedicated 20 Amp line - running to a dedicated breaker box. The damn equivalent is a good analogy, but I don't see how a 3 ft power cord would be able to make up for a poorly designed power supply - assuming the power supplies in my gear is not able to provide the supply a damn would do. I don't use any  power conditioning either. Just the change from one power cord to another one has major audible impact. The damn theory also does not explain why the umbilical cord upgrade can make such a big difference - since it only connects the damn with the gear that consumes the power - in your case this would be the river below the damn.

decibell says:

"So far I experienced the most dramatic improvement when upgrading the power cord of my CD player. Analog-like liquidity, higher resolution, lower sound floor, more dynamic presentation, better dynamic differentiation and denser tone color saturation could be easily heard when swapping power cords.

I also upgraded umbilical cords of my preamp's and phono stage's power supplies and was shocked because the impact of this upgrade was even more dramatic than the simple power cord upgrade. The overall impact was so dramatic, that I am now replacing every inch of reachable power conductor inside my gear.

Is anybody out there smart enough to understand the physics behind these effects? "

The better question is if you believe that such extraordinary mind-blowing results actually occur after these mods, why do the original audio component manufacturers neither endorse such modifications nor build them into their award winning components?  Only manufacturers/distributors/marketers of aftermarket cabling, paid reviewers and those "audiophiles" who believed the hype and took the expensive plunge, tout the imaginary phenomenons these type mods produce.  There is a reason this is true. 

"Analog-like liquidity"....really?  That's a new one.  Who makes up this stuff? 

Taking a well built item and modifying specific parts to increase performance is a common practice with tangible results. Have you ever put actual performance upgrades on a car? They actually increase the performance, go figure. So why don't the car companies make the cars with these mods stock? Who knows, but not having an answer doesn't mean the mods don't work. 

Same here. Tweak away and you will find you can change the performance of your system. Why manufacturers don't include these mods from the factory is irrelevant and a distracting question. The general consensus among those who try them is that cables make a difference. Understanding the science behind this would be very intellectually satisfying, but is not necessary to hear and enjoy the difference. Working knowledge of the biochemistry of arousal is certainly not necessary to realize how freaking awesome it is to have sex and enjoy it.

Dynaquest4 - what different power cables have you tried?