Hi ,
You can do it, leave the HDMI as default in windows and change the settings in your JRiver MC. Go to tools, options, audio, output, mode settings and in device section select your rDAC as your device, and than you can switch the inputs in your amplifier HDMY for movies and the one where you have your rDAC plugged in for music. If you play movies also wit JRiver MC make sure you go to the same steps but this time go to video and in device section choose windows system default or HDMI. This is for windows 7, I didnt try with other windows versions. Work perfect for me, I only switch from one input to the other in my amp.