No, if you connect your Sonos to the Denon with an optical cable, you're using the dac in the Denon (unless your Sonos first converts to analog then back to digital to send to the Denon but I doubt it
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). I don't think another dac will help at all, the one in your Denon is fine. If you want improved sound you need to look at your room acoustics and the speakers themselves....that's where the sound quality is better manipulated, not the electronics...dacs have been fairly invisible audibly (but some may have better specs) for some time now.