I saw an interesting article on audio illusions - if you take a bassline and have it say going:
C2, C3, C2, C3 and pan this hard left,
then take the inverse - i.e. it going:
C3, C2, C3, C2 and pan hard right

- so that the same note are playing simultaniously on both sides but and octave apart and alternating octaves left/right each beat.

People get the illusion that the bassline is bouncing between speakers, well it works on headphones anyhow, not sure on speakers!