If this is the first time you hear distortion from your M22s, it is most likely the source. You can also test other sources. You can also switch your M3s to main and M22s center and replay the movie. If it happens on M3s at any volume also, it is most likely the source.
FYI: I have a marantz receiver with volume ranges from -60 to +18. I watch movie usually at 0 volume setting. My M22s are driven by NAD C272 amp and they do not produce any distortion at that level so far. So I think it's the source.
