It sounds like you might have both the crossover on the sub AND the crossover on the receiver set to 80hz. You don't want that - you only want 1 crossover at work, not two. This would DEFINITELY cause problems in the mid-bass.
The usual starting point for subwoofer settings should be: Set speakers (regardless of their physical size) to *small*, set rec'r crossover at 80Hz, set subwoofer crossover to "bypass", or the highest possible setting if "bypass" is not an option.