Premium streaming 'spotify connect' and 'google cast'

[kb588]

Hi.

I have spotify premium and use my Android (oreo) smartphone for playing music and streaming spotify.

I have downloaded my playlists in highest quality (320kbps mp3) to my phone. I also stream in highest quality to my chromecast audio (256kbps AAC)

My chromecast audio is connected with mini toslink to my DAC which is connected to my amp with RCA cables. I use the mini toslink as it bypasses the DAC in the chromecast so I can use my own high quality DAC.

My question is this: When I play spotify using the Google home app is  shows 'devices available' and my chromecast audio as 'google cast' and not 'spotify connect'.  Is there a difference? Is it streaming from the spotify servers at 256kbps AAC? I assume it is as all the options in spotify settings are not available when streaming this way.

BUT if I set spotify to offline mode and play my downloaded playlists (320kbps mp3) I can still play to my chromecast using the Android 'play to' function and connect this way without streaming.

To get the best quality sound to my amp what is the preferred method?

Thanks.