[SteveCallas]

If your own seating position is ~6' away, then I wouldn't worry at all. Two enclosures would definitely make constructing and moving the subs a lot easier. Downfiring won't add extension per se, but I often see it associated with smoother FR's in room. I think what you were referring to is when the baseplate is close to the drivers though, like with an SVS sub. The drivers essentally couple with that air between the baseplate and it acts to lower the Fs of the driver a bit. That's not necessary for this sub though.

Make sure you are taking breathing room for the external port opening into account as well - at least the diameter of the port, preferably more.