Best quality connection from Network Streamer (BluSound Node II) to Preamp (MC C2700)

As previously pointed out, the Node 2 does not have USB output. If you output via coaxial or TOSLINK you will bypass the Node's internal DAC.  You will want to use fixed (not variable) output setting.

You can connect the coaxial and analog RCA outputs simultaneously and switch between the two and decide which you prefer.  The Node will output both simultaneously.

I have a Node 2i with a Denafrips Ares II DAC.  The DAC in the Node isn't bad, but I have no experience with your preamp.  But based on its price, I would hope it would have a better DAC than a $600 streamer/DAC combo.

So you spent 10k on a preamp and 500 on a streamer. Before worrying about the quality of the connection I would for sure upgrade the streamer.

I agree with @riie .  Upgrade your streamer to something like Bricasti M5.

I have a C2700 with the DAC2 module. Started off streaming with a Mac Mini (Roon core) and recently upgraded to a Lumin U2 Mini. Really big difference in sound quality. I tried Coaxial and USB cables between the Lumin and the C2700. The USB cable sounded much better to me. I also don’t know how much of the Lumin streamer improvement might be because of the network connection. 

That DAC2 module is decent. I have a Schiit Modi+ dac that I use for my Mac Mini. I was curious if it would sound better and the DAC2 module sounds better. 
 

I do plan on upgrading my DAC down the road. The DAC2 is convenient, but now that I have the Lumin, I really need a Dac to take advantage of the better streamer. 

I’ll agree as well regarding a stand alone DAC. For now I’d say the digital coax should be your best result. I had the Bluesound and that was my choice of connection.  
I have a Gustard DAC now.  I see a lot of posts about the Denafrips Pontus 12th. Has anyone compared to to the Gustard 26 pro.  I may be looking for a new DAC.