adamanteus, there is no "plug-n-play" method that will work with all setups, room sizes etc. For instance, I tried the near field w/both subs at the front of the room, both inside, and outside of the L/R to no avail, so alot of it will be room dependent, and gear dependent, experimenting is the holy grail so to speak.
donatelloa, for the range your looking to spend, you might be wise to include Paradigm Servo into your list, a little more affordable, but they wont be as musical. Just one sub will do good in some rooms, so its possible that a DD15 by itself would do the trick, possibly even one DD12, but like most everything in this hobby, trial and error becomes inevitable. If you could afford just one DD12, or DD15, I suggest to start there, and see if it would work in your room, or at least in the seating positions. Go from there if necessary.