It's well documented that no matter how well intentioned the "changer" that they usually can't help subconciously giving out subtle facial and other body language clues that let the listeners know whether it's A or B they're listening to.  The listeners themselves don't know that they're being influenced or how but they are.

In the case of an audio test, if the "changer" is never visible to the listeners and doesn't say anything they can hear then that's OK as he has no way of influencing them whether he intends to or not.

 

That's a bit rich coming from you Antonio!  You're always the one trying to convince everyone that your kit is the best there is

Michael.