Most Rotel Integrated Amplifiers provide both line-level and speaker-level outputs that can be used to connect a powered subwoofer to your system. 

To connect a powered subwoofer via line level (RCA), the Left and Right PRE OUT connections can be used from the Rotel unit to the line level input on the subwoofer. 

Alternatively, most subwoofers also can be connected via speaker wire, and your subwoofer’s owner’s manual should give more connection information on the various setup options using speaker level connections.