You could also run RCA's from the 3480's "subwoofer out"s to the EP500 and still use the 500's built in crossover. The 3480 does not do any filtering on the sub out signals; it just passes the full range to the RCA jacks.

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