The bigger the magnet does not mean the sub will hit harder or be louder. The entire construction of the sub is important to the final output, not just one feature.
The purpose of a magnet goes back to how a sub/speaker works. A speaker has a negative and a positive terminal. Your amp will give the speaker an AC signal to the two terminals. This signal runs through the voice coil which is simply a wire that is wrapped in a circle. This wire is connected to the cone. When the wire is powered it will act like an electromagnet. That means when it is powered the magnet on the sub/speaker will be attracted or repelled from the electromagnet caused by the wire. This will either pull the cone toward the magnet or push it out. This in and out movement of the magnet is how the sub produces sound.
So the magnet is very important for a sub as it would not work without it. There are also other parameters that are equally as important. The combination of all these parameters is what makes a sub loud or sound good.
Double stacked will refer to having two magnets, one stacked on another, instead of one magnet.
Good Luck!!!