Dange, maybe not the middle of the room, unless it is doubly functioning as a couch or overhead feature. However, a quarter wavelength from the wall may be quite practical. Also I would say the opposite would be true of membrane and panel absorbers, I presume they work best on the boundary?
I am curious to see if Traps more or less attached to the back of the speakers, 2-3 feet from the wall will diminish SBIR more than at or slightly off the front wall.
I think the front wall reflection, with it's consequent up to 20dB dip deserves particular attention. That dip is often right in the midst of a very important musical zone. It uniquely doesn't change with frequency like the side, floor, and ceiling, reflection dips. It would be good if we could all settle on a term for that one alone, rather than First Comb Filter Dip Caused By Front Wall Reflection. I can't even remember what SBIR stands for by it seems to me that most use the term to describe FCFDCBFWR
DD