The lowest you can set the x-over is that way because you've assigned speakers at 80Hz, thus it won't let you set it any lower than that for the sub - as it should be. Since you've done this, there's no reason to use the x-over on the sub itself. That adjustment is there for when you have a system that does not have bass management and need to dial it in yourself. If it has a bypass mode, use that, otherwise turn the x-over all the way up so it does not combine with the AVR's x-over.