Power cable education needed

As long as your existing power cable is not any worse than the Romex that is used in your house electrical system, it does NOT matter.  Remember, your house wiring is insurance mandated.  Unless your house was built by a cowboy, you are guaranteed to have a safe 15 - 20 amp wiring.

Wiring is like a chain.  The chain is as good as its weakest link.  Since all your house is wired with a Romex cable of appropriate size (typically 15 - 20 amps), adding a much stronger link into the middle of it does NOT make it any better.

Same with plumbing/piping.  If your whole house is installed with a 1 inch diameter piping, adding a 2 inch diameter pipe just before the faucet does NOT improve the water flow.

@johnnyaudiogoon,

Thanks for explaining your experiment.

To tell you the truth, I cannot explain the improvement you are hearing.
The only scientific explanation I can give is the fact that maybe, if your house installation is SO GOOD (ie your chain is already so thick) and much better than both your power cables, it then exposed the shortcomings of your power cables themselves.

Do you have a dedicated special high current wire in your house to the outlet your amp is connected ? 

I cannot think of any other engineering explanation :-)