The following script is useful for folks who have USB or FW dacs. The supplied Mac remote will normally not control the output to the dac. Running this script after starting iTunes enables the remote to control the volume slider bar in iTunes:
on idle
set machineVol to output volume of (get volume settings)
tell application "iTunes"
set sound volume to machineVol
end tell
return 1
end idle
Copy and paste to Applescript then hit the 'Compile' button. Then File -> Save As -> File Format “Application”, check “Stay Open”, click "Save".
WARNING: The first time you use this script, do NOT do so without any sort of preamp or passive volume control before your amps. Once you get a feel for how it works, and understand that the iTunes volume slider is mapped to the Mac's system volume, you can try going direct to your amps.