it's about 9 months I got the Pi3+, I stream music to a Beresford Caiman SEG DAC and from it towards a vintage audio system with an Audio Research SP9 preamp, a Chinese twin KT88 Aeron AP890 power amp and then to JBL 4311B speakers through old Monster Cable Powerline cables.
Question is, I would like to add a coaxial output to the Pi (my DAC has one USB, two optical and one coaxial, at this moment in time the coax is the only available and I could free up the USB for a MacMini by using the coax for the Pi) and also, eventually, to get a step above in quality, which steps would you suggest to achieve both without selling a kidney?
maybe you can consider buy HIFIBERRY DIGI+ Pro, and installed/plug to your PI. Then you can use Coax or Optic to connect to your DAC. I will choose coax over optic, but I believe usb connection will be the best specially if you play dsd.