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

It is correct that the Node II does not have an USB out. So chosing coax over toslink would be an easy pick for me.

Had similar setup for awhile. 2700 and newer Node with USB out. My ears found the USB connection to be the best sounding to me. The McIntosh DA2 DAC definitely sounded better than the Node’s DAC. I later borrowed an older Naim DAC-V1 from a friend, and the improvement over both the Node and the DA2 DACs was very easy to hear.

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.