If you watch a lot of movies, I would definitely vote for the sub first. The sub will make a huge difference in your movie-watching enjoyment. I rand for a while with Axiom M2s and a big SVS sub, which made a surprisingly nice combination.

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