About 3 months ago, I purchased an ARC CD8, which ought to be similar to the CD9. The paperwork that came with the CD8 mentioned that break-in should take up to 600 hours. So far, I have about 180-200 hours of music played on it. I haven't experienced fatiguing or harsh sounding music, however it took (still taking?) a while for the sound to open up, and the bass to be more present. I just listen to the music and try not to worry about the break-in.
Be patient, bear with it, and it ought to slowly sound better. One tweak I can recommend is that if the CD9 came with a 6550WE tube, replace the 6550WE with a Winged C 6550C. My CD8 came with a 6550WE. After changing the 6550WE to a Winged C 6550C, the CD8 is more musical, and the change is an audible improvement.