Adrien, as Peter pointed out, if you reversed the polarity of the connections to all of your speakers equally, then they're in phase with each other. If the RCA plugs have different colors, the red ones typically are the positives.
There's a minor controversy in some circles as to whether "absolute polarity"(i.e. whether the speaker cones go in and out when the recording mike diaphragm went in and out)is significant. Aside from the fact that polarity often is reversed several times in the recording process, tests indicate that it isn't audible, at least on music. Possibly the YPAO is sensitive enough to have picked up the "wrong" polarity in reproducing its test tones.
