Hi.
I had the same issue with my Saga+ preamp (and Vidar amp). Have you tried removing the tube? (The Saga+ works fine without it in place.)
I contacted Schiit support and provided them with a detailed description of the issue (50Hz hum at 60db, regardless of volume level), immediately when the Saga+ is switched on.
The support engineer responded very quickly and suggested removing the tube (the originally supplied Tung Sol 6SN7 GTB). This solved the humming noise issue.
Waiting to receive a replacement tube from them now.
I tested the grounding of the casings (using a Fluke precision multimeter). The Vidar case is properly grounded. However, the Saga+'s case is not connected to the ground pin. The RCA jacks outer rings are connected to the ground. Schiit did not respond to my question on whether the case should be checked for proper grounding... Will follow up if they do.