If you connect via Toslink the Denon will do the DAC and pre amplifier duty. If you want to use the DACs in the CA streamer you will need to connect with RCA. Try both and see if you like one more than the other. Might not be a difference.
You should use the Denon for volume control and set the output on the CA streamer to ‘fixed’ or equivalent, a doible volume control setup is not a good idea. Just remember to change that back again if you connect it directly to a power amp.
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