Your sub's x-over should be set to its highest point, allowing the x-over in the receiver to handle the task. jax is 100% correct, if your speakers are set to large, your x-over isn't in effect. A x-over isn't an all or nothing deal, it is simply the point at which the blend starts - sound is slowly reduced in the mains or sub respectively as you move away from that point, so each will still have some sound above/below the x-over point. Even with floorstanding speakers, most still cannot handle bass nearly as well as the sub, so you will likely want them crossed over. I'd start with 50Hz and see how it goes.