Almost embarrassed to ask, but...

I’m seriously considering a high(ish) end used pre amp as my next upgrade. Currently using Marantz AVP, and looking for the next step up in sq for 2 channel. I have an Oppo UDP205 for discs, but also use a lot of streaming music. The Oppo has the DAC, but I’d need to get the streaming music into it. From the Oppo I’d take analog out to the pre amp. I can use a tablet, but I’d prefer a stand alone unit plugged into the Oppo. Also, what is the best connection for digital output of the streamer to digital in on the Oppo?
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.
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

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. 
I would definitely consider the Oppo UDP205 audiophile grade piece of equipment, and is quite versatile as well.  it’s possible to stream music from your computer via the USB DAC, use it as Roon endpoint, or stream via your network using the Oppo Media Contol app.  It’s quite acceptable to use it as a preamp, however in my particular case I did not use it that way (connected it to an integrated amplifier via the XLR balances outputs.
Oppo and Marantz are both directly addressable as streamers. Both use UPNP and HEOS. I use both and often compare them. I find my Marantz 8805SE performs better than the Oppo205SE.
I've also wired the Oppo’s analog XLR stereo out to other amps Including the Marantz as analog stereo inputs, and using RCA to my PassLabs XA25.
All whilst happily streaming Tidal via the mConnect Lite app.
And I’ve used the Pioneer N70ae Streamer and a PS Audio PerfectWave DAC with Network Bridge Streaner DAC in this HT system. And earlier, a Cocktail Audio X40 Streamer.
All in, direct to Marantz via HEOS or mConnect Lite is best for using my HT System as a music player.
I have a separate stereo system with streaming, SACD and other sources.
I stream to my Oppo 205 using a lap top- USB -Roon/Tidal-into a tube preamp then into a solid state amp and it really enjoy the music!  I’ve tried the Oppo 205 direct into the amp but there is something about the tube preamp that is more statisfying.
@taheeti May I ask which preamp are you using?
I stopped in to my local dealer on a whim, just to see what he had. They suggest digital out from the 8802a pre/pro into the Oppo is the most seamless, then Ontario the streaming with phone/tablet as I do now. I listened to a PL Evo400 pre they have setup into a SS power amp and it sounded really wonderful. Reviews on that pre support my theory that adding a pre could very well improve the experience, as I’m hoping. Now to decide on the pre amp....

That EVO400 would be the upper end of the budget, 4500. I have no previous experience with tubes. I am not a hobbyist, I have no interest in the rabbit hole of tube rolling, but the idea of a tube pre is one I’m open to. Not wedded to, but open to.
I just went through something similar. I switched to Roon + Tidal. I purchased a Roon Nucleus( they are may other servers option to use Roon) that is feed into my Oppo 105 via USB, Oppo feed Into my Marantz SR7005 via analog. Very happy with results. 
I suggest you search out the thread titled preamp deal of the century on this forum. It will help you learn about the supratek preamp which is right around $2,000 and the Don Sachs SP preamp and the ultraverve preamp. These are all tube preamps and are universally praised for their sound.
I did search out that thread, and lots of good stuff there, although HT bypass is a must. 
I’m starting a new thread for the preamp options....

thanks all!!!