Which DAC?

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Thanks to some help on another thread I have decided to purchase Chromecast Audio for doing multiroom sound in the house. The question is what method should I use for connecting equipmet when I have a choice?

I will be connecting to a Denon DM40DAB, a (very) old Sony STR-DB930, and a Marantz NR1605

I am vaguly aware that you should use the best DAC to perform the conversion so I assume the best approach would be to use the Optical connection when the DAC in the amp is superior and the 3.5 analogue connection when the chromecast DAC is superior. So in short:

1. should i use digital or optical connections for each of the amps listed above?

2. Will using optical on some and analogue on others create lag issues resulting in poor multiroom syncing?

Cheers

Ben