From what I remember, the V12 is a capable sub but the X series models from Paradigm do perform better. If you want to stick with Paradigm the X15 is a good option but you will pay a premium for the brand. If you want to go with dual subs, they really should be the same make and model, though.
The X15 was discontinued, unless you know of a place that has new old stock. Maybe able to get it at a discount? Their current mid-tier is the S10 and S12. The S12 would spec close to the X12 but it's not cheap. Nice sub if you want something stylish in gloss black.
Monolith tends to be the performance per dollar winner but a more utilitarian look. Hsu, SVS and Rythmik would be the ones I would look at first. If it's an open shared space the size and look of the sub may matter, so think about how large of an enclosure you can accommodate and whether you want to go dual or single. A rough budget would also help members give recommendations as prices can vary greatly.