It means you either reached the limit of your amplifier (close to full clipping), or power compression began to really take it's toll on the subwoofer. Or, it means that you are really bad at judging increases in volume at high output levels //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif Just because it didn't "sound" like it got louder, doesn't mean it didn't. But you'd need a db meter to tell if there was any actual increase in output.
Anyways......at what volume did you set the gain? If you set the gain a little lower, it will allow you to turn the volume up higher before this affect sets in. It will not, however, all you to "gain" anymore actual output. Just allow you to use more of the volume range on your headunit before you reach the peak output.