I emailed Mick from Supratek and he kindly responded even though Supratek is no longer. His reply:
100K input impedance with the manual volume control
50K input impedance with the remote volume control
Both values are for the Chenin linestage.
100K input impedance with the manual volume control
50K input impedance with the remote volume control
Both values are for the Chenin linestage.