Using the line level high-pass outputs
If you are using a preamplifier which does not have a sub/LFE output, you can send its left and right front output into the line-level inputs and then connect the sub amplifier’s high-pass outputs to the inputs of your main amplifier.
The subwoofer will play the low frequencies and your main amplifier and front speakers will play the frequency range above the subwoofer’s fixed high-pass crossover point.
The signals coming out of the high-pass outputs are not affected by any of the controls. They are just a copy of the signals going in except that the low bass is filtered out. This uses the sub amplifier’s passive crossover network, rather than the active network and controls.
This is an excellent method if your speakers are small satel- lites or mini-monitors, and/or your power amplifier is of limited power, such as a tube amp.
User's Manual