What brand and model of preamp is it? My guess is that High and Low refer to high gain and low gain. If that's the case, the low gain outputs are most often used to drive a solid state amplifier and the high gain outputs to drive a tube amplifier, as tube amplifiers usually have lower gain than solid state amplifiers.
So I would start out connecting the low gain outputs to the Rotel amp and the high gain outputs to the subwoofers.
Steve