The speakers only cross between their own drivers. The upper and lower end are unbounded. The sub-woofer crossover stops the lowest frequencies from reaching the main speakers and sends it to the sub-woofer instead.

Have you tried 80 Hz all around? 40 Hz is pretty low for the QSs, you're likely missing some bass content that the surrounds are trying to play, and the sub isn't picking up.
Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs