Ok, thanks for clarifying. Outsiders like me usually forget how the system is configured between posts. Very often there's a dip around 70 hz due to floor-ceiling cancellation, so that could be it.Crossover is quite high: 120Hz so it's coming from the single sub and the L&R speakers contribute very little, if anything, there in the nodal region.
You find that out by measuring at different heights.
It's easy to walk into the trap of trying to boost such a cancellation, but it never ends well. Aside from boosting the cancellation with the same amount you boost speaker output, you end up with crazy peaks at the same frequency somewhere else close to the listening position, which will indeed be audible at other points in space.
Personally I feel a 70 hz cancellation is particularly bad because there's a lot of energy and drive in the music around there, so I would try to see if there's a way to avoid the issue. If I'm not mistaken human chest cavity also has the resonance frequency around that area, so important if you like the punch in the chest