In choosing a sub or multiple subs, you have to consider the size of the room and its or their projected location. For example, in my designed cabinets,for both the left and right front channels, the theoretical frequency response according to BassBox6 Pro, would be -3dB at 20Hz. Due to the fact that they are located close to corners, I get a room gain response of +10dB at 20Hz. EQ definitely has to be applied for a smoother response.
This means that a subwoofer which seems a little bit insufficient at some infrasonic frequencies based on serious reviews, might perform adequately in a small to moderate room size because of room gain and its position near a corner.