Although this has been asked and answered many times here, I'll add to the database of discussion.
There are many reasons power cords make a difference, and "what sonic difference they make" is dependent on a myriad of design factors such as inductance, impedance, shielding, dielectric, contacts, wire type and gauge..etc etc..
Contrary to the popular refrain "why should the last 6 feet of fancy power cord make a difference at the end of two miles of Romex?" The truth is that the "last six feet" is NOT the last 6 feet from the perspective of the component, it is the FIRST six feet. In addition, EMI and other high frequency noise and interference do not travel down power lines like debris travelling down stream-- most HF noise is radiated from point sources. This radiated noise loses intensity the farther you move awat from the source, and then most of this type of noise is gone, entirely.
Now, the relevant issue becomes what are the point sources of noise that because of proximity, are potentially the most significant in effecting system performance?
Generally speaking, components in an audio visual system are all within a few feet of eachother, they are all interconnected and are tied to a common AC source (dedicated lines help). All of these components have power supplies that radiate EMI noise, and all pass noise in varying degrees back onto the power line via ground. As you approach this combination of power supplies, all of these radiating feilds of energy and noise are converging.
A system of electronis often acts as a "loop" of sorts for radiated noise and energy fields that can not possibly be (entirely) ameliorated by traditional power supplies or out-board conditioning/filtration . The type and form of AC signal isolation applied in those "first" few feet outside a component has the potential to impact the condition of the AC signal as it passes into the power supply. Of course, power supplies have their own methods of dealing with incoming noise, but most manufacturers of electronics will admit, there is no perfect way (yet) to eliminate the effects of noise riding in on the power line.
Of course, there can be exhaustive technical discussions, arguments, counter claims and the like on these forums, but the bottom line is: you HAVE to try these products for yourself within the context of your own system to judge their value and effectiveness. And because the many power cord designs on the market vary widely in terms of quality and design, you should try more than just one type or model.
There are many studios, including those owned by Phillips and Sony, and mastering engineers, recording artists, producers etc. who have found after market power cords to be a valuable tool in improving their systems and recordings. Many manufacturers of electronics/speakers in this industry use and endorse them as well. I think we've past the point where there can be any question whether there are power cord designs that can make a positive difference in system performance. The only relevant issue left is proving to yourself whether a specific product can make positive difference --in your system. Forum Q and A's all become moot at that point.
Happy listening!
