Pumakeeuuh , simply ground them to mixer GND, thers no need to grund it to spin interface, or make bridges from mixer to spin.
Mixers also have better Grunding coz, they have big surface of steel/metal, and GND are connected to them.
Simple expample for it is:
You using old amplifier whit phono channel in it, and it have no grundig, just stik ground cabel, to something whit metal/steel sourface, and You see that signal have good potential of power/ground and its cleen.
So You dont need to use grounding from specifited hardware, it can be mixer or spin, either.