My unscientific testing produced no hum: select a quiescent optical input to avoid source signal issues and turn the 990's volume all the way up. I'll repeat this with an analog input once I've got everything hooked back up (tape, phono, and VHS are still on the floor :-)

Likewise I've noticed no buzzing or other electrical noise from the 7700, or 990 for that matter.

I'm using the balanced connectors and everything is plugged into the same outlet. I'll probably install a new outlet on its own circuit at some point, but that'd be for load reasons, not to solve any hum issue.