I don't think setting the speakers to LARGE will do you any good if you want a crossover at 40 Hz on the mains and surrounds routing said frequencies to your sub.

You need to set those speakers to SMALL and then set to 40 Hz with the crossover, turning on the bass management.

The double bass seems to occur if you don't use the built in 80 Hz analog crossover on the 5.1 analog inputs (for a DVD-Audio or SACD or universal player)

Dan