Thanks to the soundrecover feauture.
6400 hz is output of the naida. With soundrecover enabled, you can hear ,for example,8000 hz at 6400 hz output. Of course, sound quality changes a little due to frequency shifting but the brain gets used to it over time and everything sounds normal.
And, I had my aids reprogrammed . My frequency upper limit is now 7500 hz. But I can hear speech a bit clearer and louder.
You can test your frequency range with some online test sites. It is not professional measurement but gives you an idea.
http://www.audionotch.com/app/tune/
Before the test make sure you turned the volume of the computer to the highest.