If you're running HDMI from the PC to the Rotel, then the Rotel should be doing the D.A.C. and the PC DAC is not involved, unless you are also running line level audio out from the PC to the Rotel. The Rotel will likely have a better DAC unless you purchased a dedicated audio card for the PC for that purpose. I ran Plex on my PC to a PS3 connected to the AVR but changed to a Roon Server for audio with a Raspberry Pi running the Roon client software feeding the AVR via HDMI, since the PC is in another room. You would just need the Roon Server on your PC. The theory behind Roon is pretty interesting although the improvement in audio was hard to detect on my system. What I like is the gapless playback, controlling Roon with my cell phone, and the excellent way that Roon manages to randomly select music in similar genres. I also went through the long process of re-ripping my CD collection to go lossless, but it was well worth it. Mp3s do not play back gapless, and I find the gaps on live albums or where songs blend together to be very annoying. Roon is not cheap for a lifetime license, so I went for a one year trial but will likely get the full license later.
From what I gathered from another thread, you're likely better to stick with HDMI rather than play with optical, especially if you go to a new AVR with the latest HDMI spec. If you have any mutli-channel music tracks you'll need HDMI to decode them properly. Optical will do 2-channel ok but is lossy with multi-channel. Nice PC BTW.