If the crackling happens only at loud volumes you could be playing it louder than it can play, boosting the bass more than it can handle, or it could be a bad woofer or amp since you got it used.
Does the cracking happen when you connect it with short cable? Does it happen if the sub is plugged into a different AC outlet?
More specifically, a speaker coil becomes permanently warped due to high temperatures. Once that happens, the coil rubs against the magnet. There is no reliable way to un-warp a coil.
Other problems includes a torn spider (the part behind the speaker cone that prevents it from moving too far). Also, weakened foam surround will allow it to move too easily. This is often the case when a speaker is improperly stored for a long period of time. Downward firing subs are especially bad for this. Drivers are to be stored either vertically, or in a container that holds the cone in it's centered position.