Like any speaker, the further away from a wall it is, the less it is affected by a room boundary, and will produce a more even bass response. How far will vary from speaker to speaker, and the room itself is also a factor. Some rooms, a speaker might need a couple of feet, in another, only half a foot. The good thing about the Studio 140.2's is that they're downward firing, so this helps quite a lot. I know someone who's using his D28's about half a foot from his solid rear wall, and the D28 isn't exactly bass light!
Do you have a solid floor, or is it a suspended floor? Is the wall behind where the speaker would go, a solid brick wall, or a stud/partition wall?