It is in the speaker set section of the menu. Most people set their main, centre and surrounds to small with a crossover at 80hz (I run at 60 hz for the mains). By setting the speakers to small it allows the crossover to be adjusted. I found that I had to try different settings so that the lower frequencies would not be directional.
After the crossover menu, you can also adjust the "trim" level if you find the bass to much or too little - every subwoofer, user, and room is different. In my favorite room, I have to back off the subwoofer trim by -3db otherwise it interferes with the other sounds.
My subwoofer is switched to "auto", unless the volume is loud enough, it will not turn on.
A long time fellow user named Gonk used to have a download to use with a CD that would play the lower frequencies. I found it useful to better understand base management.
Starting at page 27 of the 976 manual it goes over the speaker settings.
Hope this helps.