I thought the idea of of 212 series was that it’s like a built in line array with vertically arranged drivers giving the bass an added height dimension. I this the new one has 4 drivers including backwards firing.
truly, if you call REL, they will recommend the right pairing for your setup. Interestingly they do not always suggest bigger (higher model) is better.
id be interested in what you select and how it turns out.
At CAF, I saw the borresen M2s partnered with their 4 driver dipole bass modules. The difference with and with the module was incredible. After hearing it with the module, removing it was quite a disappointment, even at the M2 level.

