If at all possible 3 to 4 ft should be the bare minimum. My friend has line arrays which were a couple of feet from the walls but one day we tried them 6 ft from the rear wall and the difference was incredible the speakers totally disappeared and instruments seemed to hang in mid air. I never thought that would have affected speakers that were used for mid and treble reproduction only.
Distance from rear wall for KEF LS50 W2
I had placed my KEF LS50 W2 speakers against the rear wall, rationalizing that the pair of KC62 subs took care of LF. But when I pulled the speakers out to 30" at the face of the speaker, it seemed to open the sound. Wonder what others are using or recommend.