I prefer the coaxial output more than the USB output from N6ii, even when the USB Audio output offers higher bandwidth.
The SPDIF output from N6ii is a plug and play setup, unlike USB Audio output, you don't need to tweak the output setting. I can test the Spotify output to S/PDIF later just in case.
yes, if you want to connect N6ii to a audio pre-amp, a high quality line out is recommended, so E02 is recommended. If your pre-amp has a balanced (XLR) input, the 4.4mm to dual XLR from iFi is recommended. If your preamp is single-ended only, you can use 4.4mm to 2xRCA interconnect, this will take the R+, L+ and GND from 4.4mm and use it as single-ended line out. I don't think you can find this cable off-the-shelf, you need to either DIY or custom build from technician or audio cable shops.
Thanks again, enjoy your weekend!