The SINAD covers that already. Good SINAD will certainly give good sine wave at -90db.
The measurement of -90dB was just another comprehensive test for someone who doesn't want to believe SINAD or doesn't know what it means.
Thanks. SINAD wasn't covered in that particular review so I do not have a reference point there. Just that the two DACS had overlapping performance but one had a much smoother low-level sine wave when compared.