My Oppo is a bit too bright

Try a Vibrapod 3 under each foot. I was never a fan of tweaks, but this really made my Oppo205 sound much more tonally rich.  Worth trying before going down the more expensive paths I think. 

“Any suggestions for a warmer digital sound”

Have you tried putting the Oppo in the microwave?

A more fruitful approach  would be to use an external DAC, and personally I would recommend on the does not use an ESS Sabre chip, as does the Oppo.  My Oppo 105 sounds much better as a transport into my Bryston DAC3, which uses AKM manufactured chip.  ESS have fantastic detail retrieval but can sound harsh.  I had another DAC featuring the same chip as the 105, a Mytek Manhattan, and it offered different filter options, but none of them seemed to tame that fierce treble (my speakers are B&W 803D, and those diamond tweeters can be to much of a good thing, particularly with solo Piano).  At any rate, DACs sound different, similar to variations in turntables and phono preamps.  Try something else.

An external DAC using the AKM AK4490-series DAC chip might provide the sound you’re looking for. The Schiit Modi 3 at $99 is a bargain but bested by the new Modius at $199. 15-day money-back guarantee if not satisfied.

Another option is the use of an equalizer for the digital side. The 4-band Schiit Loki at $149 might be a good low-cost solution. Same guarantee.

If you feel in a spendy mood, the RME ADI-2 DAC ($1149) with AKM DAC chip and Bass,Treble, Loudness and 5-band equalizer controls might be the ultimate solution.

As I discovered, it doesn't take much these days to get a DAC that will outperform the DAC section of the Oppo 105.  So I agree with the others that an external DAC may get you what you are looking for.  But I disagree on the suggestion that the fault lies with the ESS chip.  Some of the best DACs out there use ESS chips.  And extensive modifications of the Oppo DAC section using the same chip have done very well.  The secret is in the implementation.  You have a very good analog set up.  The Oppo is like a Swiss knife.  It does a lot of things pretty well, but can be bettered for particulars.