I have a Alpine CDA-9847 headunit. Alpine type S speakers and an Audiobahn atb10at powered subwoofer. I recently realised a very peculiar problem that not only do I not know the solution too but I can't explain VERY well either.
What I feel as it is, is a safety feature that protects my sub from going over it's limit and blowing. What happens is I'll be turning up the volume... you know, 19...20...21...22... and then as SOON as I hit volume 23 everything lowers. Low. (subwoofer), medium and high (tweeters).
It's as if they were all "deadened" I guess you can say. The high is not AS high, the mid is not as intense and full and the low, again is just less. Now what's funny is as I raise my volume more (up to volume 28+), all the frequencys start becoming stronger and more pronounced.
It's as if everything (most noticably the sub) reset to a lower power so that nothing will blow when it reaches a higher volume. Now I don't put ANY bass to the speakers so they are not trying to protect them from the speaker bass. I have tuned essentially every setting to discover a solution but with no prevail.
Where I got it installed they had never heard of such a thing, and I even got a replacement sub to see if it was just broken.
If anyone has any idea, hit me up I really want to fix this.
From bcae1.com: "The loudness contour is employed in most audio devices which have a volume control. At low volume levels, the human auditory system is less sensitive to frequencies at the upper and lower ends of the audio band of frequencies. The contour INCREASES the level of the audio signal at those frequencies (where the ear's sensitivity is reduced) as the volume DECREASES. At the upper end of the volume control's range, the loudness contour has little or no effect. "
So...at lower volumes your system will sound slightly better to adjust for human hearing, but as you increase the volume it will stop adjusting. The bass and treble returning as you get to the very top aren't abnormal.
If that isn't it, send the head unit in for service.