While I agree with what DrDennis says, I think that it applies to more subtle differences. You have to remember that when you power down your components and then power them back up, some components take some time to get back to their optimum internal temperatures.
When it's the case of subtle differences, I find it easier to allow the 'A' power cords to remain in place for about a week. Then I put the 'B' set in and do the same thing. Then I'll do it again until I find that I can tell which I ultimately prefer.
Other times it's pretty obvious after 12 hours as the components warm back up. For instance, my Silent Source Music Reference power cords versus my new CH Acoustic (formally CH Technologies) X15 power cords. With the Silent Source my system sounds quiet and very detailed with the soundstage at the speakers and back. When the CH Acoustic are introduced, the soundstage expands out and back with the soundstage itself set behind the speakers with a quieter background and more resulting detail.
Chuck