When I was selling audio equipment at Innovative Audio in Brooklyn N.Y(now in Manhattan)many many years ago, the show rooms were set up when only a single set of speakers were in the room playing at the same time. We would wheel them in and out for a demo. Anyone with 2 or more sets of speakers in their room(with only 1 pair playing), the other speakers will also play, from the air pressure created from the playing pair. Room size and listening volume all have a more or less effect on this. Put you finger tip on a woofer ever so lightly of a non playing pair while others on playing and see what you feel. If the driver is vibrating, you are listening to it. Other than multi channel, multiple speakers in a room will add smearing and distortions, to these ears anyway.... remove the non playing speakers from the room and the playing pair will sound better.