Subwoofer hook up

Perhaps a dumb question, but here goes. I'm getting a pair of small Magnepan that will no doubt benefit from a sub. I'm using a NAD C372 integrated that does not have a dedicated sub out jack (it does have pre amp outs). So what I'm wondering is: if I use the high level inputs on the sub (Athena AS 400)...where I run speaker wire from the amp to the sub, then from the sub to the speakers....will the adjustable crossover in the sub filter the signal going to the Maggies? In other words...if I set the xover at 80hz, will that prevent the Maggies from seeing lower frequencies? Thanks for the thoughts.