For streaming Spotify and Tidal will a DAC improve quality over a Cromecast

Currently I'm streaming Spotify/Tidal using a Cromecast connected via an optical cable to a Rotel rsx-1550 which I'm using as a pre-amp and a McCormack DNA-225 amp which is connected to Vandersteen 5's.

My question is if I'm streaming from spotify and tidal would there be any appreciable gains from buying a DAC over the Cromecast on my current system? IF so which DAC would ya'll recommend? 
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if you've connected the chromecast w/an optical cable, you're already using the dac in the rotel and not the (probably inferior) internal dac in the chromecast. if you're not happy with how the rotel sounds, you can of course get an external dac--if you specify a budget you'll get many recs
@loomisjohnson thanks that answers my question. I'm very happy with the sound. It makes sense that the optical would just port over the digital signal to the Rotel which would then translate it.