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.
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?