Best upgrade for cd player?

Want more nuance, detail, dynamics.
My recipe is as follows YMMV. Tube preamp for nuance.  For bass and dynamics plug into high current outlet of power conditioner or straight into wall socket.  For more details use 14 gauge or smaller power cord. 
@gshepardbuster . Thanks. I use high current power conditioner :)   (wall power horrible)  . Spectral pre. Thinking of lps for digital section of oppo 205. Wondering about clock upgrade. Very nice movies w oppo. 
Lps will likely improve sound over a switch mode.   My Meridian 808 had the switching power supply but now is lps.  Still sounds better with high current outlet, go figure. 
Depending on the cartridge, a VPI classic can out-dynamic, out-detail and out-nuance even a really good CD.   

But I think the OP might be looking for a DAC.  Use the existing CD player and coax connect into the DAC.   Lots of used DACs here, on ebay and elsewhere. Some of those are probably reasonably satisfying.  But that they are for sale doesn't speak very highly of them since it seems their owners weren't satisfied with their performance and wanted something else.  

Get a good DAC.  That’ll give you more of an improvement than pumping more $ into an Oppo that’s out of business — good luck when you have a problem.  What’s your budget?
I can tell you that a bettrer cord and interconnects can make a difference as well as a dedicated conditioner that separates the digital from the analog equipment as well as a dedicated line to plug all your gear into.
Audiolab cdt 6000 transport and DAC like chord qutest will whip the opposition of 4x the price. Too scary good for the low price
If you get a good DAC then you you don't need to do anything to the Oppo. I have a Halo May DAC with the 203 and an I2S to HDMI converter that I can play all my cd's through including SACDs. Works and sounds great!!!
Don’t take offence , the very best upgrade hands down to my EMM Labs player was replacing it with a Innuos Zenith 3 server .
There are a number of reasons why a decent server is far better then a disc player , sound quality is one reason. 
An external DAC can add much, as can power cords on both. I added a Benchmark DAC 3b to a Sony XA 5400 ES and am very happy with the results.  Audioquest cords for both.
Agree with richdirector. The Audiolab does what a transport is supposed to do; with my Neko Audio D100 MKII DAC, it sings. Better than with the Chord 2Qute DAC I had previously. Read on the Neko site on why it sounds so rich and warm.
@dino & midafeff1. I use an MIT Oracle power cord which I've found to be good and I agree with you the pc's are important. 
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. 
Why don't you let Modwright do it's thing on it?  Really, I had an Oppo for a while and never could get used to the CD sound.  They are going for such good prices, if you don't modify it, why don't you sell it and buy a better player? A friend of mine with very good ears replaced a Linn Klimax server with a Modwright 105 and much prefers it.  I didn't hear it myself. 
+1 on the Audiolab 6000cdt and a good DAC. Mine is paired with the Schiit Gungnir MB. The results have been very good.
I don't think there is one for nuance, detail and dynamics.

All CD player sounds are notoriously close and differences are subtle to say the least. Try comparing a budget machine to a top of the range one.

Or even a Blu-ray / DVD one.

On the other hand a careful loudspeaker upgrade might well give you all 3.
I have an Oppo 205.  A modest improvement can be had by (1) putting it on sorbothane feet, and (2) opening it up and putting dynamat on bare metal, and especially on the transport.  
There are a lot of aftermarket power supplies.  They seem easy to install.  I am skeptical but cannot say as I have not tried one. 
Don't go with snake-oil cables.  However if your pre-amp has balanced inputs then using XLR cables will give a very small improvement. 
My personal experience with sorbothane is that it sucks the life out of music. Check out Machina Dynamica springs, which I have under everything but my speakers, which are on Harbor Freight dollys

There’s a lot more to balanced then just having XLR I/Os. The circuits must also be balanced= more money=better sound

Get a good DAC.  That’ll give you more of an improvement than pumping more $ into an Oppo that’s out of business
+1 get a good new school R2R discrete dac with volume, and go direct to poweramp/s

Cheers George
+1 in_shore. Computer/server audio surpasses sonically my CD transport (Teac/Esoteric P1). Having my CD collection on a hard drive now, as well as streaming Tidal via an I Pad is just the icing on the cake. Good listening!
@jafant . If I had funds would buy reference dac, likely Berkeley.  Accident/medical expenses killed that possibilty. Off hand answers to simply go out and " buy reference" are a bit difficult for me to appreciate; plus they are not really on topic. Are you becoming a little cynical? :) 
Not at all. I still believe in the CD/SACD spinner. That is all.Take it or leave it.
Happy Listening!