+1 on adding a sub and it doesn't have to be the exact same sub. More on that later ...
The ting the OP needs to keep in mind is that adding another subwoofer is not a walk-in-the park, plug and play , solution . It will take time, lot of time, perhaps additional components.. Yes , unfortunately, it requires that you spend time on forum to unedrstand how and what to do ... People here are willing to help and they will if you ask ..
This said ..
One more subwoofer (PB-1000 or PB-2000) and
REW software (free)
miniDSP UMik-1, measuring microphone, not free: $125 on Amazon
miniDSp2x4HD, a DSP/box that would help immensely integrating the 2 subs... $325, last time I checked...
Lot of time to learn to measure, Measure, Learn to apply, Apply, Learn stoopid interface, Apply said measurements to stooooopid interfaces Ask, here, We will answer.. Be frustrated.. come back here again and be satisfied. rewarded .. And then, with the F206 in a medium room, subwoofers correctly integrated? Heaven!
Peace.
Mini