As for grounding, on more recent compressors I made sure that the chassis was bonded (as they say) to the ground pin on the mains socket, the audio ground was left floating and only connected to the ground connections of the inputs and outputs. This is the classic grounding system that works in most instances.
Back in the days of the SC2.2 it’s possible that the grounding was less carefully done.
I suggest that you make sure that the chassis is well connected to the mains ground pin.
In smaller installations it’s generally OK to have the audio ground common with the mains ground, but there is then the danger of ground loops and consequent hum.