I didn't like how the system volume wouldn't change smoothly by default when you turn it up or down with the PowerMate. It goes a "block" at a time, just like if you hit the volume up or down buttons on your keyboard. When you control just the iTunes volume, it's smooth; but I wanted to control the system volume so I could adjust it for Spotify, YouTube and whatever else makes noise.
Side note: Why are so many volume controls not continuous? Isn't that just better?
Anyway, with some AppleScript, I made it change smoothly, just like the knob on a good old-fashioned stereo. Enjoy!
set therecord to (get volume settings) set theoutputvolume to output volume of therecord set volume 7 * (theoutputvolume + 2) / 100
set therecord to (get volume settings) set theoutputvolume to output volume of therecord set volume 7 * (theoutputvolume - 2) / 100
By the way, here's a really helpful post on how to get the PowerMate to intelligently play/pause iTunes or Spotify, depending on which was last playing.