… But looking at your waterfall plots and realizing that the time scale is different on the before and after graphs; you actually improved the situation surprisingly considering the very small area treated and not even on a related surface area (for about 50 Hz) if panel is lying on the floor …

I´d say your room is good enough to benefit from 2 subs and a DSP to be able to correct the low end (and to be able to x-over above possible floor related SBIR dip in the 110-170 Hz or so range) with decent results.