It sounds like you have installed a ground loop into your system. Describe the signal path to the Dyns through your system. I would start by disconnecting your Dyns from the 7700 altogether and see if that helps. If the Dyns are self amplified I am not sure where the 7700 fits in to your system. How is it that you "disengage" the 7700 when listening to the 110As? I would think that the DAC output would go directly to the Dyns amps. I assume you have the DAC connected to the 990s digital output for volume control. If so, connect it with the Optical connection vice the coax to eliminate any chance of that ground loop.

The manual definitely wants an IR connected in zone 2 to the IR input on the 990. That connection should not cause any ground loop issues at all.