I'll leave the x-o for Wayne to respond to, but we run a Genelec 7070 sub with Four Pi mains. The mains are not high passed, so I parallel the main CR out to the main monitor signal chain and the sub. For me, using a sub with alt. mon nearfields in pointless, because I want to hear the limited low end un-enhanced. When I want full range I use the mains. We have shitty NS10s for nearfields as well.