With a superb DAC like the RME I’d be tempted to let the RME DAC do any up sampling needed for analog waveform reconstruction. It will do the best job possible
Basically let me restate your question: " Do I want to process the music file in Volumio and then send it to the DAC for a 2nd reprocessing" There is no right or wrong here it all comes down to how it sounds. The pro side is to send the best digital file you can to your Dac and let it process it from there. The negative side is ANY processing by Any product is never perfect, as they are all best math estimates of the manufacture. Therefore the question comes back to process once or twice it all depends on how it sounds to you. Listen to what you hear not what you read.