Yeah, you'd get bass cancellation and everything. It's not the rear cones extending more due to the sub, but more the cones being extended by the sub during reproduction of it's own midrange to low frequencies, which would then be canceled out. This is an issue that can cause more headaches than loud bass itself in a 5.1 home setup. You can have beautiful bass everywhere around the house and living room except right at the listening position in the middle of the room. Especially when all speakers are nicely angled at the listener, all canceling each other out. I've experienced this.

The speakers aren't meant to cancel out. two waves placed on top of each other superposition, creating a louder sound, unless out of phase. This is ofcourse in a perfect world, but still no cancelling should be occuring. Not in a house.

Cheers

I think what I experienced was the room boundary cancellation effect. Waves hitting a wall, reversing and joining with waves from the source.