Just a few generic responses come to mind-
First, treatment is typically independent on the kind of monitors wether near field or otherwise. A near field response would seem to have advantages in that there would likely have be less influence from the room. However, sitting position is often dictated by the size of the room so that a larger room- further sitting position is often aided by the more forgiving aspects of a larger space.
Regardless, yes, you probably need quite a bit of treatment in that very small room. You may benefit from room measurement software such as REW, however, generic treatment advice will likely give you several steps in the right direction. Lot's of great info in the "stickies".
You may have phase issues with your sub out of alignment with your monitors.
Lastly, you wouldn't need any treatment with headphones.