The distance from the rear wall just affects the amount of bass reinforcement. Some people find the mid-bass a bit boomy if the speakers are too close to the wall, but (a) it definitely depends on the room, (b) something like 15" seems to be the largest distance anyone has needed and most run them closer.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8