Most Rotel Stereo Preamplifiers provide at least two line-level pre outputs, one of which could be used to connect a powered subwoofer to your system.
To connect a powered subwoofer via line level (RCA), one set of the Left and Right PRE OUT connections can be used from the Rotel unit to the line level input on the subwoofer. The other pair of PRE OUT connections would be used to connect to your main power amplifier for your speakers.
One word of caution - ideally you should choose a subwoofer that has both a Left and Right line level input. If your subwoofer only has a single mono input, just connect the Left or Right Pre Out to the input; do not use a Y cable to combine the Left and Right Pre Out as this can result in the output of the loudspeakers being output in mono instead of stereo.