I had the same issue when I set mine up. This thread pointed me to the solution.
https://community.roonlabs.com/t/chromecast-audio-why-dsp-volume/47345
Turn the volume on the Chromecast all the way up and then control volume via the device. I can confirm this got me bit perfect output with a CCA connected to a Peachtree DeepBlue 2 speaker. Let us know how it goes. Cheers!