Hi do you have a single listening position or multiple listening positions? If you have a single listening position then you can probably be fine with one sub and eq for that position. But when you get into multiple positions to have good seat-to-seat consistency a second sub would definitely be helpful.
The frequency response measurements you have shared are with smoothing? If so would it be possible to share ones without smoothing then you can see exactly how much is being lost in terms of gain and also the smoothness of the response with the second sub.