Learned something new today and it isn't good.

Apparently this practise was developed as a way of reducing the signal to noise in digital audio.

Well, it was to INCREASE the signal to noise.  As I recall it had something to do with the early anti-aliasing filters not being great.  AFAIK, any CD player should automatically detect and enable/disable the matching de-emphasis as should most  audio playback systems.  However it's imperfect.  According to this, it's not on a lot of CD's so maybe you don't care??

https://studio-nibble.com/cd/index.php?title=Pre-emphasis_%28release_list%29

Pre-emphasis was a way to reduce tape noise from analog recordings.  When studios started recording digital there is no use for it.  CD contains pre-emphasis bit engaging de-emphasis built into every player automatically.

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.