Audio-gd's usb input is not as good as it's I2S or Coax/Toslink. 
But you can always use any kind of DAP with digital out(DX100 or DX90 or AK1xx) as spdif transport for the Audio-gd which improves the sound by quite abit.
And once you tap the I2S connection for the Master 7, then you have attained R2R Nirvana(Summit-fi level).
The advantage of M7 over Yggdrasil is that M7 has jumper tweaks that you can do to customize the brickwall filter and oversampling settings to your liking while the Yggdrasil is not tweakable.
So far I have not heard of any Sigma Delta Dacs that can sound as realistic as the R2R M7. 
R2R is really on another higher level of refinement, like your grandma's homemade apple pie/cake, full of texture, crummy and flavourful. With R2R, you get to hear quite alot more in the reverb, decay and subtle tones that can be "missing" or "very hidden" with sigma delta.
A must own if you are seeking the ultimate in natural sounding music.