Quattro, what OS are you using in the Xbox? I believe if you go w/ Gentoox you can just use the Xbox controller. Not 100% though. However, if you plan on using XBMC or XBMP, I don't think there is any external way of varying the volume w/o using the headunit. However, that amp looks to have volume control built in, so that may be the way to go...I hope I helped a bit.
"I'm a dick!" "I must seek knowledge and it's bastard son truth" - The State
Do you mean an optical input to the controll you are looking for or do you mean what you said optical output
the Audiobahn ADD51T is a 5.1 processor for cars it has spdif and optical in it is the only 5.1 unit for cars that has a volume controll and does not require a analog rca signal to controll volume you can't have digital and analog at the same time with 1 sound card so this unit is the only way to go