Everything in the signal path makes a difference.  What we hear and experience is in a very direct way the AC power itself.  The AC power is directly in the signal path. As an example, as an amplifier transfers energy to speakers it draws down storage capacitors by exactly the same amount. The capacitors try to keep fully charged by drawing energy from rectifiers that turn the AC into DC from the transformer that draws power through the wall. Ultimately everything you see, hear and experience is the AC that must be identical to an amplifier’s output, and that AC travels through the power cord! That’s why power cables are as important as any cable in your system because they carry the same signal.  The same is true for source and preamp equipment.  Upgrading the power cables can bring a very real improvement to your your system.