Those AVM20's are about 10 years old now, and minor upgrades to DAC's over the years have happened, whether or not those upgrades are audible is up for debate.
I wonder if there is something a miss with your ripped music as even FLAC files have been known to be missing the odd bit here and there. Some of the newer ripping programs verify the rip against the master to ensure there were not any mistakes(a long process), perhaps this is what is causing the sound differential and not the DAC. It is simply that you are hearing the true sound from the cd.
