Turn Itunes Down when not active window

I know you can turn itunes up/down using Ctrl+Arrow up/down but it doesnt work when its not the active window.

i cant turn my volume down using Dinova keyboard because i'm using Toslink which doesn't allow you to turn it down.

Setup - Imac -> 3.5mm to Toslink - > DAC -> Amp -> Speakers

what i want to do is Turn down itunes whilst playing TF2!

any ideas?

i tried adding a keyboard short to - & + keys it didnt work.