Don't think a USB DAC can be used on Apple TV. I've used the ATV with excellent results thru a Benchmark DAC1. Unless you are up to doing serious mods, it's probably best to use optical out.
***On a side note, I read in a forum about a couple audiophiles who built a capacitor filter fob that plugged into the USB plug and added additional filtering to the 5v line. Apparently both the toslink transmitter and usb share the line so the extra capacitance on the 5v line contributed to better sound out of the toslink output. Basically it looked like a thumbdrive, but contained capacitors instead and only used the 5v line, not the signal lines.