Almost embarrassed to ask, but...

My source is the Oppo 103 and I have tried it as a pre going into a Vincent SP-331. Ultimately I have things set up now using the 103 digital out into a Schiit Modi Multi DAC then to a Schiit Freya (the one with the tubes) then to the Vincent amp. Very pleased with the sound, but that's with my speakers, my choice of cables, and in my heavily treated dedicated listening room. As always, YMMV.
Tom

Don't know what your budget is but you could check Backert Labs

The Marantz is a processor, not receiver, but the room is dual purpose home theater, so Oppo to amp isn’t an option. Amp is Odyssey Stratos, speakers are Aerial 7T, and I’m confident the sound for 2-Channel can be improved with a ‘real’ pre amp, not the avp. The Oppo has no streaming built in, though, hence the question. Although.....could I use the preamp that does have Spotify built in as a source to the Oppo, as well as the Oppo being the source to the processor for movies? I’m thinking that would create some kind of loop though?

In almost all cases, coaxial is better than optical.
Just get a pro audio 75ohm mogami cable (or any pro audio brand) and you are good. check sweetwater.com

preamp is a touchy subject. At the top level systems, in most cases top preamp will best a volume-control DAC (even top level there are exemption like emm labs new offerings). But low to mid-tier, if the processor/dac make good emphasis on the volume control (keywords like discrete components, class-A high bias output stages...). Oppo UDP205 probably use digital vol control with op amp. It's not going to win award.

Maybe a Hegel P20 or Benchmark LA4. The Hegel has dedicated Home Theater bypass inputs.
The Oppo 205 has usb inputs as well. I have a Oppo 203, so can’t add much additional to the conversation.