The USB connection will only support two-channel PCM.

As for whether the PS3 can output audio (PCM or otherwise, any number of channels) over USB, I don't have a PS3 to experiment with either but I've not heard any hints that this may be possible. My suspicion is that audio output from the PS3 to the 990 via USB is not possible. If I'm wrong, though, it'll be limited to two-channel PCM.