Strongly encourage you to move the sub first. Then it’s pretty easy to view the problem using the right tools like Room EQ Wizard and a calibrated mic or OmniMic.
If you don’t want to move the sub, you might try EQ route first.
Measure the sub alone first, and clip the peaks. Then try to get a descending slope from 20Hz to 100 Hz or so, about 1.25 to 1.5 db/octave. Then set the crossover frequency for smoothest transition.
If you had not said space was at a premium I'd suggest corner bass traps/soffit traps as great additions. Maybe your next place. :)