Wouldn't you want to use the same clip? The most important thing science ever taught me was that you should only change one variable in an experiment, otherwise the results could be skewed by the new variable. In this case, only the cable would be changed, and you want people concentrating on that one clip. If there is a difference, you want people to take note of it, and get used to the clip. Its hard to compare say, Pink Floyd music and John Williams, to be able to see if there is a difference in cables.
The only instance you want to change the clip is if you rerun the whole experiment, with the same cables and order as you did the first time, only using a new clip. Then you would see consistencies/inconsistencies between what people hear.
