replaced my Fireface800 with an HDSPe AES card. 
Everything else in the system is identical except for new monitors.
the converters accept ADAT and AES. 
Now I have a low level ground hum. 
If I swap the input to my stepped attenuator from the equipment racks to a separate laptop, the ground buzz goes away.

All of the equipment, the monitors, and the computer runs through an AC voltage regulator and power conditioning.
has anyone else seen this?
The obvious difference is the move from fiber to copper connections between the computer and the equipment rack.   But I'm open to any other suggestions.