I have found greater sonic differences in the streamer (or Roon endpoint), than resulting from the server (Roon core + local files).
When comparing, I think it is important to understand what the device is being used for and in the case of the Oladra, K50, Grimm, and others, those are performing the dual functions of both server and streamer and outputting directly into a DAC. Units like the Innuos Pulsar, Auralic Aries 2.2, and Sonore Rendu line-up are stand-alone streamers (only, no server), although the Aries 2.2 (that they call a "Streaming Transporter") has local file storage capabilities.