On my older system an active electronic crossover box came with my Infinity Modulus system. RCA output from preamp out to Infinity box, and RCA’s from Infinity box to power amp. The box has it’s own cord to the powered Infinity sub. Have it set at 90hz with volume level adjusted by ear. With my newer system the speakers run full range (no active crossover) and the upper range of the subs is currently set at 50hz