I tested underclocking-volting the C1.
The best setting was suggested by another user 2 weeks ago:
675/200 530W 803Mhs 0.66W/Mhs
760/250 790W 1008Mhs 0.783W/Mhs
FYI I have an EVGA 1300W gold.
I tried 0675V/200 - working but with x
With 0700V I never had any x on the same machine.