I would think long and hard about getting dual subs. I would try to get subs with good upper bass response. Then having each sub near respective monitor will give you the most flexibility in tailoring your xovers, even if only to have the freedom to choose a higher xover. The dual subs will also even out your room modes.
You could always just add one, see how you like it, then if you wanted to, add another later. But, if I did it all over again, I would take two slightly-lesser subs, instead of the one excellent sub. I think anyways. Hm.