Subwoofers do not cause vibrations in the room due to the interaction of the cab with the floor, or, more accurately, due to the interaction of its rubber feet with the floor. If this were the case, manufacturers could eliminate expensive drivers. The bass is produced by the driver, which transmits sound through the air to its surroundings...the floor, ceilings, walls, furniture, people, etc. A piece of foam placed near, on, or under the sub will do nothing other than fool people into believing they hear some magical difference. If the feet are too stiff on a hardwood floor and the sub moves around a bit at high levels, some foam pads under the feet for $1 will do the trick. People used to put butter on burns because they "believed" it was soothing. Doesn't mean it really helped.