There are other threads discussing this if you search for them.
Flat frequency response of +/- .1 db over the full range and SINAD of 120 db will do it. Probably SINAD of 110 would do it.
With flat frequency response lesser SINADs would be transparent, but you have to break apart SINAD into distortion and noise. For instance flat FR, noise below being heard for a given system and THD of -80 db will likely do the trick. Such a device however would only have a SINAD of -80 db.
Probably why no one has replied to you. It quickly gets into plenty of grays areas.