Hi @Mikex 

Welcome to the Sonos Community!

The Sonos Connect has a perfectly good DAC of it’s own, but if you prefer your external one, you’ll need to wire one of the digital outputs (TOS-Link optical or co-axial RCA) of the Connect to the input of your DAC. The analogue output of the DAC should in turn be connected to a line-level (not Phono) analogue input of your amplifier.

I recommend you set the volume of the Connect to “fixed” and control the actual volume via the amplifier’s control: Settings » System »  room with Connect] » Line-Out Levels » Fixed, though you can also try Passthrough to use the Sonos app’s volume control.

I hope this helps.