Learned something new today and it isn't good.

Please don't ask me to explain this in any more detail as I will already be pushing the limits of my knowledge. Use Google if you want to learn more.

Nyquist rate DACs have a zero-hold structure. They are not flat to 20KHz and roll off towards 20KHz. A zero-hold means it holds the same value between sample updates. I understand ADCs don't normally have this issue because they take a snapshot in time. To compensate for this roll-off, the CD could have pre-emphasis to compensate and get a flat response. The roll off is not very big. When up sampling DACs became standard, this was not needed as the roll-off at 20KHz was very small.

The redbook CD standard has a flag on the CD that says pre-emphasis was applied. The CD player should read this and tell the player what it needs to do if anything.

Pre-emphasis is the first part of a noise reduction technique in which a signal's weaker, higher frequencies are boosted before they are transmitted or recorded onto a storage medium. Upon playback, a de-emphasis filter is applied to reverse the process. The result is a higher signal-to-noise ratio (SNR); the original frequencies are restored, but noise that was introduced by the storage medium, transmission equipment, or analog/digital conversions is quieter than it would have been if no filtering had been done. Pre- and de-emphasis can be collectively referred to as just emphasis.

Emphasis was sometimes used in digital storage media in the late 1970s through early 1980s, including on a small percentage of audio CDs. Emphasis is akin to Dolby noise reduction for tapes, or the RIAA equalization curve for vinyl records.

From what I gather emphasis was implemented to try to improve the sound of the CD players, not the cd's themselves. This is a case where the media was better than the technology to play it.

Pre-emphasis is very likely dead IMHO, because ripped CD's data doesn't carry pre-emhpasis bit. A lot of people use servers and music with pre-emphasis would sound bad (+9.5dB@20kHz) without de-emphasis.  Some ripping programs might contain plug-in to de-emphasis if needed. There are programs (like SoX) to de-emphasis digital data, but it is additional hassle (to find out if it is needed and convert every song).

(In common CDP to DAC setup  S/Pdif carries pre-emphasis bit to DAC)

I have not played a CD disk that I would have suspected Not having such de-emphasis.  Like I hear playing a record without RIAA de-emphasis.  Or Dolby encoded tapes.  It's not broken.  You don't have to deal with it.