I hear some minor distortion in soms tracks with replay gain disabled also. So I am not sure this is the same issue.
My general opinion about the NAD M33 is that is a formidable amplifier, DIRAC limited is doing a very good job (my UMIK is faulty, this is why I had dissapointing results) and my unit is stable up to now. What I like is that it is quiet easy to turn subwoofers off and have the fronts fullrange and corrected by DIRAC. The sound (my Epicons are "full range") is slightly better with subwoofers, but very doable without. When I listen to an album that causes distortion I simply turn off the subwoofers and I am fine, but still expecting NAD to solve this issue. Still, yet, very dissapointed in NAD. My Lyngdorf gear never had any distortion errors.