I have tried to fix this problem by restoring the volume levels to where one would expect them to be. To achieve this, I have used the score, other Furtwängler recordings of the 9th, and my own judgment. This by itself makes listening to this recording a new experience. In addition, I have also adjusted equalization. The result is amazing.
This is from Henry Fogel's review of this transfer: "Chibas has managed further meaningful improvement. One area is dynamic range he has painstakingly compensated for the compression on the original source, and given us a fuller dynamic range than anything before. He has also achieved his aim of a more present timpani sound and a sense of rhythmic crispness that exceeds prior versions, without losing the beauty of orchestral sound that is a hallmark of the conductor's work. The timpani at the outset of the finale, and the remarkable presence of the double basses, make significantly more dramatic impact than I have encountered on any prior version. This may be one of the most significant of his transfers, and Furtwängler collectors might start here to determine how they will react to his work."