Dangerous is right. Even a simple fender bender could turn nasty with a speaker box, or parts of one, flying around your passenger compartment. Death by flying speaker box is not all that uncommon a death these days.
Not to mention, some of the function of your subwoofer system is to transfer low frequency material to the vehicle's chassis (to be felt). Velcro not only will not allow a good solid connection between the box and the chassis, even worse it will tend to isolate the two.
In short, using velcro will be dangerous, and diminish the performance of your system (a bit). Just bolt the **** thing down.