You forgot to mention where the music files came from, and in what format. The more detail you provide the better.
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Your Oppo has asynchronous USB, making PC hardware irrelevant (as long as driver is good). I can only guess, that computer injects electrical noise into Oppo. I would try USB cable without power wires (GND, +5V). You can also try different USB cables or add USB isolator. Plugging computer into different outlet might help. (in my home outlets are divided into two phases).
I don't know Foobar2000, but there might be gain adjustments, equalizers or upsamplers, that need to be turned off (eliminated). Another possibility is that your digital music file rip from CD was poor (error correction code approximates samples when CD is scratched or laser is dirty). Also make sure that you compare in exactly same conditions at the same time of the day (same amount of ambient electrical noise).
DCaudio - I have a similar setup, my music server is a Intel Nuc with Plex music server connected to my preamp by Hdmi. Music ripped from CD in Flac.
I also have a Oppo 203. My music server, pre, and Oppo are all connected with network cable so I have the option and occasionally play via DLNA. Unfortunately it sounds a little better using DLNA, I say unfortunately because using Plex I can sit and browse my music on my music server on my TV and play what I want. Going through DLNA I don't get the nice interface and info about the bands and music but instead a whole lot more scrolling.
As I learned recently from another member on here I can significantly increase the sound quality from my music server by; getting a Linear Power Supply, and a better quality hdmi cable like Wire world. This would probably put the sound quality on par with DLNA.
And you can also get a Linear ps and better fuse for your Oppo that will make the already excellent sound quality even better.
All that being said I am content with the current sound quality through my music server but may at some point go ahead and get a linear ps and better hdmi cable.