Almost embarrassed to ask, but...

i would use your oppo as a preamp (connected via analog outs directly to your power amp)--it's a high-quality piece that will likely outperform alot of separate preamps. you'd connect your streamer to the oppo thru the streamer's optical or coaxial out--which will sound better is a matter of opinion.

The Oppo has a good dac what amplifier are you using? Does the Marantz have an amp or is it just the preamp processor? 

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?