If the mains are set to large, then there is no crossover being used (the bass management ignores any crossover point specified for the speaker when it is set to large). This applies to any speaker -- mains, center, or surrounds -- whether the sub is set to on or not.

Double bass can only occur in two instances: 5.1 direct analog input with the bass management switch set to off (crossover is still used to re-direct material below 80Hz to the sub, but speakers get full-range signal), or stereo bypass mode with the mains set to small (the speakers get a full range signal because the signal is bypassing all processing, but a copy of the signal is sent through digital bass management and the sub gets the info below the specified crossover point for the mains).

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