the sub will move in and out.
(unless your bass note is 20hz and it's 1/20 of a second long, but it's not)
in phase or out of phase, you're just perceiving it to move only one direction.
EDIT: here's a good test.
quickly touch a 9V battery to the sub.
you should see it more in or out.
then reverse the polarity of the battery, it should move the opposite direction.