I'm surprised the M2's ADC SINAD is this low. According to Motu's website, the M2 and M4 should perform the same is this regard:
https://motu.com/en-us/products/m-series/m2/specs/
has THD+N (unweighted) at -129dBu EIN
https://motu.com/en-us/products/m-series/m4/specs/
also has THD+N...