Just as a confirmation: i take it that the M20d is NOT supposed to operate as a "normal" Audio Device - ie, if i choose M20d as the Default Communication Device in the Windows Sound > Playback Devices panel, and then i start Windows Media Player or any other music player on the PC, the player complains that it can't play out the music. Playback does not even start.
I mean, out of the USB interface, from the PC into the M20d.
If i choose Helix instead as the Default Device in Playback Devices, the Helix does receive tha audio stream over the USB link, and the WMP does not report any issues.
This makes me think any audio from the PC to the M20d has to go necessarily via the PC analog audio out and into the M20d analog AUX ingress. This is also hinted at in the M20d user guide, in the Streaming section.
So Helix and M20d are not equivalent for audio playback over USB from a PC.
Thanks for any confirmations on this point, obviously i would think the audio is better sent in digital form from PC to M20d over USB, as i would not rely on the audio quality of the internal sound card in the PC if this was working.
My PC has Windows 10 64bit, and updated M20d drivers; the M20d has updated firmware.
On the PC, in the Sound > Playback panel, i have Helix reported as "Speakers - Default Device", and just below this line i have the M20d as "Speakers - Default Communication Device". I don't know if this difference is just a descriptive label, or if it indeed means the two devices do not posses the same capbilites as playback devices.
In Device Manager > Audio inputs and outputs, both Helix and M20d are reported as Digital Audio Interface, and both are labelled as "Speakers" too...so here they seem to be equivalent in the way Windows knows their capabilities.