My brother-in-law's basement has about 18 feet (and a pool table) between the back of the couch and the back wall. He is being told to go with in-ceiling speakers for rear surrounds but my guess is that conventional rears on the back wall would still be better.

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