Thanks for the replies fellas. Setting the Sub vol control already down to the minimum does not seem to allow for any headroom adjustment on my Marantz receiver. It's pegged at -12db.

Good idea, maybe I will try to raise the sub volume control just to see if the sub is louder or if I get an error message from the Audyssey calibration. This will help determine if the volume control actually works, right?

Initially I thought it might have been due to placement of the sub in the corner of the room, but its current placement in the middle of the room along a wall still does not allow for bass adjustment.

Any other ideas?