BlueSound Node 2 - connect to pre-amp direct or to Oppo UDP-205 and then to pre-amp

Though the integrated DAC in the Node is good for the price, I would output to a better DAC, if possible.
So, yes, use the Oppo.
B

In theory the OPPO DAC should sound better but I would just try both options.
You never know and only your own ears can decide.
Good luck

Try both, but you may decide there's not a lot of difference. I tried my node 2 through my OPPO 105D and it does sound a little better, but not much and I like not having to turn on the OPPO and the TV to listen. 

I bought the Node2 a few months ago to better the sound over the Sonos, which it did. at the time I was using a Rega DAC. However I did for about a week run Node2 directly to PreAmp and it was much better than the Sonos. And thru the Rega DAC it followed suit. I recently traded the Rega in for a Chord Cutest.
Sorry I took the long way around your question, so yes the Oppo should sound better, I also have the Oppo Bd105 but have chosen a separate stand alone DAC.