If the subwoofer's LINE IN uses RCA cables and the subwoofer out on the receiver/amplifier also uses RCA, plug it in using an RCA cable. If the cable is split on one end (a y-cable for both right and left channels), plug it in the R and L ports on the subwoofer. The best way to hook up a subwoofer to an amplifier with no dedicated subwoofer out is to use a "Y" splitter cable.
Easy Steps to Connect a Subwoofer to Speaker Level Outputs
Check the specifications on your receiver.
Check the specifications on your subwoofer.
Use two speaker wires to connect to the front and back of the sub on the left and right audio channels.
Adjust the filter on the subwoofer to clarify the audio.
Enjoy the bass
While a powered subwoofer (active subwoofer) does not need to be connected to an amplifier since its amp is built-in, you can connect a powered sub to an amp for data transmission only.