Minus 3db,
I listen in the near field and my chair is fixed in one position so I can't answer your question as far as it applies to the SMS-1 in my room, I just don't move around the room during critical listening. My guess is that it will sound about right anywhere in the room except when you are near a room boundary (In a corner or against the wall). These are really long wavelengths we are dealing with so there may be dropouts in certain places in the room but I wouldn't think position would be all that critical.
My tweeters on the other hand require a very precise listening position.
Thanx,
Russ