The optimal solution depends on what you want; do you want all audio for all content below 80Hz sent to the sub? Or do you just want LFE channel and the bass component of multichannel sources sent to the sub?
If you want everything below 80Hz to go to the sub you'll need to select output format 2.1, enable JRSS mixing, and then set the appropriate bass management settings for the channels in room correction. If you want the latter, set output format to 2.1, enable JRSS mixing, and then set the appropriate subwoofer setting on the output settings page.
If you want to handle the bass management manually, set output format to 2.1, disable JRSS mixing, and then do all the routing in the parametric EQ blocks. The 2.1 setting is actually 2.1 in a 5.1 container by default, so you can use the SR and SL channels as dummies/holding channels if you need to do complicated mixing.