Buy a DAC if you're serious about Stereo/Hifi.
This is the answer. You don't want to use Apple's DACs anyway.
I wonder if USB would work as a secondary audio output?
Sure, it can be corrected with DAC converters, buying powered speakers or another powered amp/receiver (using Airport Express)...but it will cost you more $$....lessening the advantage of ATV 2 vs. ATV1.
But to be fair, the DAC (such as in a nice Denon receiver) is useful for a lot more than just with the ATV.