Your Mac Pro and the Marantz 30n are both examples of music servers. You don't need dedicated hardware to make a music server - just a computer with the right software (or the right options enabled). You can make a music server with a Windows PC, a Mac, a Raspberry Pi, etc.
The only thing that matters with respect to music servers is that it gives you the features you need with the type of convenience you're seeking, e.g. DIY or plug and play, integration with streaming services, etc.
FYI, the Marantz 30n has an interface - the HEOS app.
Also, don't worry much about cables or boxes that regenerate signals (unless you have to run USB cable longer than 5 meters). If the data isn't reaching your DAC intact, you'll hear pops, ticks, and dropouts during playback. Otherwise, you're fine.