As an aside, I also use SoundFlower, but my use case seems to suggest DPS isn’t actually doing its own volume control.
I have a multi-audio device set up to play on both built-in and a bluetooth sound card.
When I set the multi-audio device as output device, volume control does not work. OS X doesn’t even let me change it.
When I set DPS as the output device, and the multi-audio device as DPS’ output device, volume control _appears_ to work, in as far as, OS X lets me change it up and down and the DPS device’s volume changes, but the output volume doesn’t _actually_ change. When I turn the volume all the way down, the output _does_ mute.
When I set DPS as the output device, and SoundFlower as DPS’ output device, and I connect SoundFlower to my multi-output device, volume works just fine. When I increase volume on OS X, both the DPS and SoundFlower device’s volume change, and the resulting output volume goes up or down accordingly.
I cannot attest as to whether the volume change is accurate in dB or doubled as suggested by dbm, in my last scenario; but the fact that the output volume doesn’t change when SoundFlower is not in the mix suggests to me that DPS is not doing its own volume control (I wish it did!).
What it looks like to me, is that DPS doesn’t actually do any volume control and instead just forwards or syncs its own volume control on to its output device, and relies on that device effecting the volume.
So; from my place, I wish DPS could do its own volume control, the way SoundFlower obviously can, and it didn’t forward its volume requests on to its output device.