That's just wrong! I don't even know where to start on this. Maybe I should talk about the 75 ft of Romex ($0.10/ft) in the wall between the outlet and the breaker box and how impossible it would be for the last six feet of power cable to make a difference...but I'm not!
Power amplifiers have capacitor banks to filter the DC voltage rails that come after the rectifier inside the amplifier. The amplifier is using the power from these capacitors, the incoming power from the wall through the $500 power cable, through the transformer, and the full wave bridge rectifier keep the capacitors charged. Once the incoming power is removed the capacitors start to discharge. In the mean time, the music is still playing perfectly fine! With many amplifiers, you can just pull the power cable right out of the wall and it will just keep right on playing fine for several seconds or longer!
So, let's think about all this for a second. The $500 power cable can not make the incoming power better, all it can do is not damage it in any way. Let's assume the cheap power cable that came with the amplifier does damage the incoming power degrading your sound quality. How badly can it damage it?
We know we can unplug the amplifier from the wall and it will keep playing fine for several seconds. So, I'm going to sit behind your amplifier with the $500 power cord and unplug and plug it back into power outlet on one second intervals. You are not going to hear any difference in the sound. It's physically impossible! (Assuming of course the intervals allow the rails to keep the signal below clipping) So, exactly what is it that the cheap power cord does to the incoming power that is worse than this?
High end audio is full of snake oil and high end power cables have to be one of the best examples of this. It simply doesn't make sense! The power cable to the amplifier has no more to do with the amplifier's power supply performance than the pumps pumping water into the water tower have anything to do with the water pressure on your garden hose. The two are simply not connected! The pumps filling the water tower only effect your garden hose when they fail to the point that the water tower level drops significantly. The same is true for your high $$ power cable.
I know, the money is burning a hole in your pocket but spend it on something that actually makes a difference!