Loud hum on single ended inputs to CT-2300

[rghanbari]

Folks

I'm getting a VERY loud hum when trying to connect to the RCA single ended inputs on my Classe CT-2300

I'm switching out my setup from and XLR DAC (Oppo HA-1) to a single ended one (eventually a Chord Hugo 2). In prep, I've been trying to get the setup working with my Chord Mojo (single ended output)

I have shorting pins installed between pins 1 and 3 on the balanced inputs to the 2300.

Even with the Mojo disconnected from everything and powered off, I have a very loud hum when I connect to the headphone out on the Mojo. Same thing happens when connecting to headphone out on a powered off Pono and a powered off iPhone.

I have swapped cables and changed electrical outlets for the Classe, and have the same behavior.

Any thoughts/suggestions for how to diagnose and fix what is going on?

Thanks in advance

Ray