I'm not endorsing this approach, just thinking you might give it a listen and see what you think.
You may have already tried that. I am a bit OCD and knowing you have all that great woofage sitting idle is working my nerve (j/k, but not entirely
).
You need to go to Setup > Speakers > Manual Setup > Bass > LFE + Main
That will enable the Subs to play the Main content using the Crossover frequency as a low pass filter for the subs.
You may have to enable this setting before you can access the crossover setting.