It sounds like your subwoofer is passive, not active (i.e., it does not have an amplifier built into it). If it only has speaker level inputs, then the only way you can use it is with a speaker level connection. If your passive subwoofer is designed to be used with your main speakers, and if it has stereo speaker level inputs & outputs, hook up the inputs to the passive subwoofer to the front channel right and left speaker wire from the receiver, and run speaker wire from the subwoofer to the front right and left speakers; there will be no direct connection between the receiver and the front right and left speakers; the connection will be through the subwoofer. On the receiver, you will want to the settings to be: subwoofer off, main right & left front speakers large.
Now, if your subwoofer does have an RCA type input, you would probably want to do things differently.