Sorry George, Not sure where all of your info comes from, but adding mass in alone does not add distortion... In fact if the mass is a stiffener, depending on cone, it could decrease it.
@stevepa Steve, George and I could take you down 2 different roads. To be fair to George, I do not have his direct experience with your speakers, but in fairness to me, I come from a back ground of driver and speaker design. I speak from experience and an understanding of how all of this comes together.
The good news is that I do agree, that a good sealed woofer is the way to go. I have not found one that is really a great match for your Monoliths.... On the other hand, I have found 1 Dayton Pro, that would work very well indeed if you would be willing to port. It would be a very viable alternative. It seems George is questioning your measurements, if they are correct, you are at 2.5 cu ft.... Bean bags would decrease it.
I hope all of this helps, Tim