I use tube amplifiers and did not hear any difference in power cords. The power cord is far away electrically from the DC power supply. In fact the power cord is not directly connected due to the power transformer is magnetically coupled. An electrically noisy mains may cause some issues. But, with sufficient filtering in the power supply design even electrical noise on the power distribution can be filtered out.
Perhaps expecting an improvement may cause the listener to hear an improvement. The human brain is amazing. So, perhaps you can hear an improvement due to expecting it with a positive mood in your brain.
