Feedback has typically been in the 2500 - 3500 Hz range. That is also where most intelligibility is in human speech. In the older days a notch filter was used to reduce feedback but all it really did was reduce the amplification in the most important area. No feedback but could not understand as well, not a good trade off. The sound recover suggestion is certainly a good one. You have no usable hearing out at 6 - 8KHz so I would not even try to amplify that area.