I have tested the VMS4 on the MAC and duplicated this issue
In previous testing we found an issue with newer MAC OSs 10.5.8 and higher where the audio gets out of sync at 44.1kHz. For the output audio, we resolved this by setting the audio for adaptive mode instead of synchronous.
Now the problem is for some strange reason the MAC won't work with this change on the input audio.
All of the distortion issues go away when I set the output audio for 48kHz. Please try updating your unit with the attached firmware.
I'll talk with the guys at ADJ see if we want to make this a permanent change to the VMS4 firmware. The problem is the Windows driver we did is for 44.1kHz so for now only do the update if you have this issue with a MAC computer.