I can't see how a cables' position under or on the floor could possible make a difference. In my mind, having it under the floor(which is what I am doing), is preferable, to lying on the floor. As Stringreen said, is it near any AC wires?
Wow, I wish I only had 2.5 meters between the speakers and amp.
Wow, I wish I only had 2.5 meters between the speakers and amp.