Whatever sounds best to you. Generally, you should set the xover to slightly above where your mains start to roll off. Are your mains standmounts, therefore requiring the "small" setting? If they are floorstanders you'd be better off with them set to "large" unless you have room interaction problems. Assuming they are standmounts, you don't want to set the xover much higher than 80hz as the bass will become directional (if it isn't already). Bottom line tho, it's really whatever sounds best to you.

Enjoy your new sub.