No,with your Marantz crossing to the sub below 100hz, the sub crossover should be left all the way up so that it doesn't interfere with the receiver crossover. Setting it around 100hz would, if anything, reduce the bass.

Still puzzled; your usual corner doesn't sound ideal, but it should be decent. Is the other corner behind the speakers usable and any better? Is the corner near you that you mentioned trying okay after you turned the sub volume down?

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Enjoy the music, not the equipment.