First you need some more digital headroom since you are mixing signals in the digital realm. You also need to have the subwoofer channel 10dB louder than the rest. However, when it is mixed to both speakers it only needs to be 4 dB higher per speaker. Acoustically the signals won't be exactly coherent so adding .5 dB extra helps. This will take care of both:
- Adjust the Volume -10 dB for all channels except the Subwoofer
- Adjust the Volume -5.5 for the Subwoofer
Next you should protect your main speakers by setting a high-pass filter
- Remove Low Frequencies (High-Pass Filter) on Subwoofer at frequency where your main speakers start to rolloff
Finally add the Subwoofer to the left and right channels
- Mix Channels - Add Subwoofer to Left
- Mix Channels - Add Subwoofer to Right
To test you go to Tools > Advanced Tools > Audio Calibration > Pink Noise at -20 dB > Play.
Add these two Mute items at the beginning of the Parametric Equalizer DSP.
- Mute (R, C, SUB, SL, SR) - This will only play pink noise in the Left speakers
- Mute (L, R, C, SL, SR) - This will play pink noise in both speakers, but it is the rerouted LFE channel
Both Mute options should playback at the same volume. I would leave the high-pass filter off for the testing so the signals are the same.