Bevan,
I've heard that deep bass info is mono on most recordings. I tried a single OB sub for a while, and any time I turned it on the soundstage collapsed somewhat down toward the sub. It doesn't happen with dual subs near the speakers. Plus, at some point you'll grow tired of monster baffles, and when you do, you'll have to cross higher, making dual a requirement anyway. You may even want to try main drivers that can't reach that low regardless of the baffle size, again making duals required due to the higher XO. Lastly, you may just want to experiment with 1st order XO's, which work quite well with open alignments, especially when you have trouble integrating with only an XO on the sub.