Find a recording of pink noise (Mono), then place the speakers approximately 1m apart facing each other, then connect the one speaker out of phase, play the pink noise for about 4hrs or so (Be careful with the volume so that you don't overload the speakers, as the out of phase noise cancelling effect will make it seem quieter than it is) and you will be good to go, after that experiment with positioning to get the best performance. (Don't forget to reconnect the speakers back correctly)
Bill