OK, that's cos of your grounding scheme. Leave that for now. Instead, connect the black probe to the ground of the SSC board -- for instance by sticking the probe, if it fits, into the 0V of the crimp connector (on left side of board).
P.S.: If interested, for grounding the GSSL you could start read here for example:
https://groupdiy.com/index.php?topic=31116.0. Question : Audio ground connected to starground, YES or NO ? -- And if connected, then only at ONE point to avoid ground loop hum).