Here is what I used to do to set subwoofer volume when I had them. Pick a frequency that both speakers are capable of reproducing as low as possible. I use 100 Hz. Reverse the connections on your main speakers and remove their input from the high on the crossover. Set the the crossover at at least that 100 Hz. Run 100 Hz signal to both. At a volume where you would normally listen on the main speakers, now set the volume on the subwoofers to give the lowest volume. You are using the out of phase speakers to counter balance each other for minimum volume at 100 Hz.
Reverse the leads on the mains and put them on the high outputs of the crossover. Enjoy. I usually found a slight reduction for the subwoofers from this setting was perfect.
Reverse the leads on the mains and put them on the high outputs of the crossover. Enjoy. I usually found a slight reduction for the subwoofers from this setting was perfect.