One Sub does not have the power to load most rooms down to 20 Hz at least up to 15 inches. An single 18" sub might get you there. Two 12's is the minimum. Two 15's even better.
Speaker frequency response is taken near field usually at one meter from the speaker. 4 meters back in a 16 X 30 foot room is a much different story. In order to get the right "feel" you need a rising response from 100 Hz down to 18 Hz. That takes big drivers and a lot of power. Two subs will also smooth out the nodal behavior in the room. Without proper low end you will never feel as if you are at a live performance.
Speaker frequency response is taken near field usually at one meter from the speaker. 4 meters back in a 16 X 30 foot room is a much different story. In order to get the right "feel" you need a rising response from 100 Hz down to 18 Hz. That takes big drivers and a lot of power. Two subs will also smooth out the nodal behavior in the room. Without proper low end you will never feel as if you are at a live performance.