The crossover on the subwoofer is really only used if you're routing your main L/R speakers through the subwoofer because your receiver does not have a subwoofer pre-out. This isn't the case for you. The crossover will be set within the receiver, so you can just leave the crossover on the...
The smart-aleck answer is "Exactly as much as the sound engineers intended.", but I know that's not what you're getting at. As Robert_J suggested - the best thing you can do is start by properly calibrating your system using it's built-in tools. That should provide a solid "baseline" that...