Optical Rendu into Iris DDC: Will it work?

I am running my Optical Rendu into a Terminator II DAC using a USB cable.  Without considering the Optical Rendu, I have purchased an Iris Digital to Digital Converter which is now in transit.  Will it work?

When setting up the Optical Rendu  I set the output to MPD/DLNA which I expect should be fine, but I also have to use the Sonicorbiter "DAC Diagnostics" to set the DAC to MAX volume to "prevent the dreaded double volume control from happening."

I don't understand the "dreaded double volume control" problem.  I control volume with the Coda integrated amp .

Any help would be appreciated.