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.