Marantz AVR user here. I use mine as a pre-amp and stream Spotify and Radio Paradise via HEOS as well. I bought the Cambridge CXN V2 a year ago to see if it would be an improvement over the Marantz DAC. It has better detailing and sound stage when playing my digital library of FLAC files and as the DAC for my CD transport. I use the CXN when I'm doing critical/focused listening but default to using the Marantz & its HEOS app when I'm using Spotify to stream music through out the house.
I think if you have higher-res digital sources a separate DAC may be worth having in the system and for critical listening but for casual listening of Spotify premium, I think the Marantz will serve you well.
The nice thing about the Marantz AVR is having the Audessey software to do room correction. Which I never thought I'd take advantage of but do.