Question/Help Needed: Boomer who is totally inexperienced with DACs and streamers.

Question/Help Needed: Totally inexperienced with DACs and streamers. The streaming I currently do is with an Alexa device connected to the AUX input of vintage receivers with RCA cables (I have several setups in the house).

I just bought a NAD C375BEE Integrated Amp that has a DAC module installed. It is driving Thiel CS 1.6 speakers. (I am slowly trying to go from low-fi to something a bit better) 

https://nadelectronics.com/.../c-375bee-integrated.../

https://nadelectronics.com/.../MDC-DAC-2.0-USB-Digital-to...

I want to "stream" but have no idea what is needed in a streamer to deliver really solid quality music. Is it the source? The streamer? The DAC? I do like the "voice control" I get with the Alexa devices I have.

Any/all help or advice is much appreciated!! Thanks!!  John