Where do cables upgrades have the most impact?

Looks likes theres no consensus . Best bet is to buy a cable and try it out on all your components . 

Factors that tend to increase the criticality of speaker cables:

-- Low speaker impedance.
-- Long cable length.

Factors that tend to increase the criticality of line-level analog interconnect cables:

-- High output impedance of the component driving the cable.
-- Use of unbalanced interconnections.
-- Long cable length.

Factors that tend to increase the criticality of power cords:

-- The component being powered has an unregulated power supply and draws large amounts of current that fluctuate widely. Namely most power amplifiers and integrated amplifiers.
-- The component being powered tends to generate significant amounts of electrical noise, that may couple via the power cord to other components. Namely power amplifiers, integrated amplifiers, and digital sources.
-- AC line voltage at the particular location that is lower than nominal. ("Nominal" being 120 volts in the USA).

Phono cables and digital interconnect cables involve complex issues that I won’t address in this post, but their criticality will vary widely depending on the specific equipment that is involved.

Regards,
-- Al
 

the sound of your system is the sum total of all the pieces.

As a newbie with very nice cables (but with OEM power cables) there is something I do not understand about power cables. Why does a manufacturer make and sell multi-thousand dollar amps with exotic power supplies but with power cables that are sub par. I just have a hard time getting past that idea.

I've got a Proceed HPA2 amp and when I read about all that goes into its power supply like two transformers (1 per channel) and know what these sold for new I have a hard time understanding why they would send it to you with a power cable that some here suggest would be de facto inferior.

I'm not suggesting that the right power cables can't make a huge difference, but if they can, why don't they come with the unit?