Oh, don't listen to some of these guys, unfortunately the ol' knee jerk "how can the last few feet" is pretty unoriginal but thats mostly what you will get.
Oh come now. If you don't want to take the suggestions that've been offered here, how's about what Chris Russel of Bryston has to say when I emailed him some time back?
Thanks for your interesting inquiry re power cords. I can answer fully with normal engineering considerations and simple iinformation: There are likely to be 50 miles of cable and half a dozen or more transformers between you and the power plant generating the power that your sound system uses. It is unlikely, to say the least, that a power cord occupying the final six feet of that 50 miles can do anything positive to change the conditions of that power.
It is possible to do damage to power delivery of course, by restricting current flow for instance. That would be the case if you had a cord which was too thin for the power needs, for example. If the power cord was much larger than necessary, however, it would unfortunately be limited by outside factors, such as the size of the wiring in your walls, or outside your house.
It is also possible for a power cord to incorporate bandwidth-limiting components, to restrict the influx of RF or other contaminants to the 60Hz waveform. It is not possible, unfortunately, to do this without restricting the flow of 60Hz current to some degree as well. Most audio products, especially including Bryston audio components, contain internal circuitry designed to eliminate these contaminants within the power supply. They are designed to operate from an unrestricted source of 60Hz current, and will actually work less well if the current is restricted from the outside.
In sum, it is certainly possible for a six-foot power cord to have an affect on the sound, and to change the sonic quality, for the worse. It is not possible, unortunately, for this change to be an improvement, at least in connection with Bryston products. Bryston recommends that the customer do careful listening prior to spending what is usually a very substantial amount of money on exotic power cords. Remember that it is in the cord vendor's direct financial interest to proclaim an improvement in sound quality for their products, and that any change is not necessarily a desirable one.
I hope the above is helpful, but please do not hesitate to get in touch if you have other questions.