Is it possible that your system would prefer the external high pass filter being set one setting up or down from the actual input impedance of your amplfier? Sometimes one step change can make a huge difference.
Also, try the volume control in the opposite direction from your current setting. As you move up on the efficiency scale, I think the volume is lowered as you are telling it to mate with higher efficiency main speakers ... see what it sounds like as you tell it your main speakers are less and less efficient.
Also, try the volume control in the opposite direction from your current setting. As you move up on the efficiency scale, I think the volume is lowered as you are telling it to mate with higher efficiency main speakers ... see what it sounds like as you tell it your main speakers are less and less efficient.