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.