Not only is there reflection noise and variable voltage/frequency, but power companies often send data through the power lines as well. There is a pile of noise on the line. Digital components tend to spew significant noise back into the power supply as well, particularly on the ground.
Now unless you are running a floating balanced output stage in your power amp your speakers are connected directly to the main grid on the negative side. The negative speaker terminal in your power amp is on the wall side of the mains transformer. The power amplifier is merely modulating the main power grid supply.
With regard to power cables, if your power lines are run in a loop through the house, then a power cable is seen as a a spur off that loop. My understanding is that if a power cable is too short then the pc will behave as if it is part of the loop, whereas if it is say 2m or more it will behave as a spur. There is a difference.
Now unless you are running a floating balanced output stage in your power amp your speakers are connected directly to the main grid on the negative side. The negative speaker terminal in your power amp is on the wall side of the mains transformer. The power amplifier is merely modulating the main power grid supply.
With regard to power cables, if your power lines are run in a loop through the house, then a power cable is seen as a a spur off that loop. My understanding is that if a power cable is too short then the pc will behave as if it is part of the loop, whereas if it is say 2m or more it will behave as a spur. There is a difference.