The GaN 1 only has coax input. So if you have a source that has usb only then you need a digital to digital converter to change it to coax. Your source would need volume control. You can use the volume control in a lot of programs like Roon, etc. Most computers do not have a coax out....so if using a computer or Ipad then you need a DtoD converter to get it to coax to drive the Gan1. The Hifi Rose streamer has usb input and coax output (volume controlled)....but then why would you want a computer if you are using this streamer?
The GaN1 is the DAC.....it changes PCM directly to PWM (class D analog switching). A normal DAC is not needed. Less is more. Basically, a digital amp is a power DAC.....a DAC that can drive your speakers. A regular DAC changes PCM to a low level analog signal but then it needs to be amped up by an amp to drive the speaker.
Most A2D’s do not have a digital volume control. You would still need to run the converter into a streamer that has volume control.