I'd fielded the question to Benchmark. They did get back to me.
But unfortunately they didn't really answer the question I was asking. They keep saying to just set the volume to 100%. But I already know that.
Seems no one is really answering the question I am asking.
That is simple. Using WASAPI why does one dac bypass the system volume and the other does not?
Could it that the in the other DACs driver implementation it is setting the system volume to 100% internally somewhere which is invisible to the user.
It's very likely that it is up to the device or USB receiver whether it ignores the volume control or not.
The DAC2 does not seem to ignore the system volume control, even in exclusive mode.
Its ASIO driver also operates in a non-exclusive manner too, where audio sent to the WDM driver will also play at the same time as ASIO if the sample rate matches.
This is why I don't use the DAC2's WDM device at all (it's normally disabled) and send all system audio to another sound device, which is then connected to the DAC2 via an optical cable.
This provides two advantages:
1. There will never be a conflict between Media Center (or other exclusive apps) and system audio.
Many games will crash immediately if you try to launch them while MC has exclusive control over the audio device for example.
2. It means that both Media Center and system audio can play at the same time to separate inputs on the DAC. (D1 and USB, so that they are next to each other)
This lets me quickly switch back and forth rather than having to stop playback to release the device - which often requires that the other application, such as a web browser, be restarted too.
What I'd like to see from Benchmark would be an updated driver where we can control the DAC's internal volume via the PC, rather than using the remote.