Danmarchio: the speaker distance from the wall to the back of the speaker should be 2ft or 60cm, the maximum distance should be 4ft or 121.91cm from the wall depending on the listening area. The more closer you put your speaker the more coverage and bass output it will have.
I have the 1723 THX and have them 60cm from the back of the speaker to the wall for optimal bass output.
hope this helps