i have an harman kardon avr5000 as a second soundsystem.
when you set your mains at large they play below 40hz. when you set your mains as small you can choose at what crossovers setting your sub will take over (60hz, 80hz, ...) the low frequencies wich your mains can't handle. So in your current setup you have choosen to let play your mains frequencies till 40hz and lower than that your sub takes over. Which isn't a bad setup.