I actually have this too... I'm having some issues with it, I just need to sort them out. Like, the 4.1 doesn't work... it just plays it in 4.0 so I get so sound to my sub, then when you play it as 5.1 it thinks the sub is a center channel and a sub, so it plays both... and I don't know how to fix it.
The volume knob controls the master volume, this is actually how I have mine set up.
It's mounted upside down and wired to the back, so I can actually use the volume knob right there. I painted it black too...
Another problem I have with it, is that it doesn't always connect on startup, either takes a few restarts, or you just have to unplug it from usb and back again. Kind of a pain.. but yeah.
(I know the fab job is crappy

it's all redone now though)