The capacitor in series trick is normally used with sealed cabs but this does look like a good case for what is known as a fifth order alignment. It will shape the response below 200Hz, adding a couple of dBs between 80 and 100Hz and filtering below 50Hz. The cab will sound fatter and power handling below 50Hz will improve. I'd suggest a value of 300uF in this particular case, using a 100V bipolar electrolytic.
While the capacitor should help, it can't perform miracles, as it only reduces the power going to the driver at low frequencies by about 3dB.