T, just a quick partial reply. Keep in mind that the "+ main" setting just adds more input to the sub; the mains remain playing full-range when they're set large. As to how much of the mains low bass(not LFE)also goes to the sub in that setting, the Denon manual isn't clear(surprise)as to whether an arbitrary frequency(e.g. 80Hz)is used or one of the settings from speakers set "small" is also applied for sending mains bass to the sub.

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Enjoy the music, not the equipment.