This bass management thing seems to have gotten a bit tangled-up, maybe over terminology. If the mains are set large they play full-range(no crossover frequency applies directly to them)and none of their bass goes to the sub unless a bass setting termed variously "LFE+ Mains", "Double Bass", etc. is selected. When that's done the low bass for the mains channel is sent to the sub as well as the mains. Redirected low bass from the center and surround channels below the crossover frequency or frequencies isn't affected by the "+" or "Double" setting and so is played by the sub alone and isn't doubled into the mains. The mains would be responsible for the low bass from center and surrounds only in the absence of the sub(and of course are automatically "Large" when no sub is set).
