if you have the rear speakers off with the subs going and just the pressure from the subs is moving the 6x9's, then its possible they could be damaged, but it'd take a lot to do that. you'll prob just get some distortion. you could put them in a box if you wanted, they make 6x9 boxes, not really sure on air space though. a lot of people will just take out the rear speakers, leaving the open holes for the subs to come through. really makes a difference if you can't have the back seat down and takes some pressure off the trunk, which will reduce rattles.