can you please explain how to read this graph? What is it we are looking for? What's good and what's bad?
Ideally in an HT arrangement your graph should look like a straight line with no ripples and should gradually be rising from 70Hz down to 20Hz. This is called a House Curve. Think of it like the roof of a house.
Your sub's response should, rather, MUST be aided by the room gain such that the low end is naturally raised. This is room dependent and looking at the plots you've made thus far shows your room is offering very less.
While ripple free response is what all the bass-heads shoot for, it is not possible in real-world coz of room acoustics. But, +/-3dB is very much possible and that comes with careful placement of the subs. Rarely do acoustics and aesthetics go hand in hand. You may have to compromise on aesthetics for sub placement in order to get +/-3dB.
50.1Hz is in the middle of 0Hz to 100Hz passband. So if your SPL meter reads 80Hz at 50.1Hz, it should be either +3dB (83dB) or -3dB (77dB) on either side to have a flat response. Visually it will be ripples, but sonically it would sound to have same SPL across the entire passband.