All wire, all metal, can act as an antenna to RF...all antenna are metal arranged in a manner to receive a bandwidth that the designer had in mind. In that same vein,
wire can be designed in a manner to reject/cancel unwanted
frequencies. The less RF and other noise introduced to
an audio components power supply or chassis give that
component an easier time in doing it's job. As in waveguide
theory....quater-wave away from an open, is a short. A powercord design can do a good or poor job at being a short
to RF...ie: an antenna..or an open/choke/block. What we're
trying to work on here isn't 110v 60cps. You'll have to
look to higher level HZ., or your own ears. But again, if
there is nothing heard in an upgrade powercord, then it
isn't worth ones money.