Interesting spanner - my feeling is that I could add a dozen Wharfedale subs and still not get much 20hz action. The other major constraint is space, although it's a large room, it's already full; I only have space for one extra box, and I'm not so wedded to audiophile stuff that I'm going to start getting rid of other furniture etc etc.
So, my current strategy is to replace the current puny sub with something much more capable - a BK XXLS400 or a Monolith Plus. I haven't found any independent measurements yet, but the paper specs look fine.
bkelectronics.co.uk
bkelectronics.co.uk