Stephane, I've pondered this for a few minutes, but the reported effect is puzzling. The floor to ceiling room mode is actually at its minimum midway between the floor and ceiling, so this primary mode, which would be around 71Hz for an 8' ceiling(1130/{2x8})would actually be a bit louder when you sit down. The magnitude of the effect is especially puzzling. Ponder...ponder...ponder.

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Enjoy the music, not the equipment.