I have no idea why you decided to upgrade to a new equivalent crossover without upgrading your drivers...you appear hopelessly confused:dont-know and while your predisposition with vibrations is interesting its not the point of the exercise.
My expertise in vibrations extends well into the sub octaves
, Pooo!
The 2121 diaphrams are very expensive and less robust then the 2426 diaphrams. The latter is still much better than the 2420 dias, smoother and sweeter. Its the best improvement along with a 2122 recone for your system.
I have used the 2426 diaphrams from some years now and you never hear me complain.