However, since the subwoofer is not powered, I'm not sure RCA will put out enough power to drive it. It sounds like the subwoofer is designed to be hooked up to an amp. So you'd go RCA from your receiver to the amp, then speaker cable from the amp to the subwoofer.
AV receivers never have amps for subwoofers because there is no way they can know what the impedance and power requirements the passive sub would have. No point in charging for a 100 watt into 8 ohm subwoofer amp when the subwoofer needs 1000 watts into 2 ohms. A powered sub takes care of that and no one ends up paying for inadequate, excessive, or if they don't have a sub, power they don't need.