The muffling of high frequency sounds could be due to a problem in 5.0 where the Digi 001's inputs 2, 4, 6, and 8 are delayed by 1 sample, and outputs 1, 3, 5, and 7 are delayed by 1 sample. This can cause the waveforms of a Left/Right pair to phase out at certain frequencies (more prominent in the upper frequencies).This problem is solved in Pro Tools 5.01.