OK...I did some troubleshooting.
* When there is no connection to the Kali RCA inputs, there's no hum.
* When the Kali's are connected directly to my desktop Mac or when they're connected with the iFi Nano in line, there's hum.
So...clearly a ground loop. I know from audio experience that ground loops can be notoriously hard to track down and eliminate.
Any suggestions?
(I don't recall this being an issue with my Tannoy Reveals and this Mac, but that doesn't help me now.)