Have you considered rebuilding the box and using the two speakers in an Isobaric configuration mounted face to face ?? When you run speakers arranged isobarically, you effectively cut the Vas in half, and since you are now running one speaker in the box, the effect is similar to inceasing box size by a factor of X4. What does the data sheet say about running one sub in a 4.4 CuFt box ?/
You can still run both speakers in parallel. If you mount them face-to-face remember to reverse the polarity of one speaker so the cones are moving in sync.
You will see much larger cone excursions than you are getting now. This isn't an ideal solution but it might be worth trying, if you want to trade output for extended bass.