Best upgrade for cd player?


Want more nuance, detail, dynamics.
ptss
I thought you were looking to improve the player you had.  In that case, the Oppo 205 is a really good DAC. I had one - bought a Bryston BDA3 because it takes the DSD stream from the Oppo and I have a lot of SACDs, and honestly couldn't hear much difference between the two.  I think at this level you may want to figure out what kind of sound you want. IME the 205 is really transparent and detailed, perhaps can be a little bright.  In that same vein I liked the Lumin T2 better.  If you want warmer, perhaps the previous suggestions of tubed DACs. I moved on to a Marantz SA-10 and that was a good move towards smoother and more musical. But getting an outboard DAC is a great option, there are so many choices.
The Denafrips Venus ll should be a significant improvement in every parameter, or you can also probably do better on the cheap with a Gustard A22 or the new Soekris DAC2541.  Just a few to chew on, and best of luck.