Learned something new today and it isn't good.

I have an old tube DAC that has de-emphasis circuitry and an indicator light when a CD has pre-emphasis.  Only a few classical CDs in my collection made this indicator light go on; maybe I don't listen to those anymore, but I haven't been troubled that my subsequent DACs lacked this circuitry.

If you think you want a DAC with the de-emphasis circuitry, I'd be willing to sell my Anodyne ATAS.  It was working fine last time I used it.  When I bought it in the 1990s, it sounded better than a well-reviewed Wadia DAC I compared it to in my system.  The ATAS cost $3000 then.  I've intended to put it up for sale, but just haven't gotten around to it.  IIRC, it only goes up to 48kHz, so it won't work for today's high-res files.  But it will give CDs some nice tube warmth. 

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)