Have read here and elsewhere that early CD players weren't
transparent. The usual reason given is they used complex
analog filtering at the output which resulted in audible artifacts.
Later players lacked these complex high order filters and are
considered transparent to the source.

My question is in the normal specifications, early CD players
tested out with extremely low distortion, flat frequency
response etc. etc. In short the specs look pretty much
identical to those of current CD players. So what part of
what kind of technical testing would reveal the audible
deficiencies of those first generation players?

Dennis