Do I need an audio card with USB output, or is there a way to route the PC audio through the PC USB ports?

The 990 is the USB audio card. All you do is connect a USB cable between the 990 and the USB port on your computer and Windows will install a USB audio device driver.

You will need to make sure that the windows driver is selected as the default audio device in Windows Control Panel.

Has anyone checked if it is possible to send a bit perfect stream this way?

Bit perfect is not possible cause the windows driver resamples to 48kHz.

I was aware of K-Mangler resampling, I was hoping someone had a work around such as a kernel streamer.

The USB driver does not support KS.

This may be a silly question, but does this mean that Dolby Digital and DTS streams can be output by a pc (and decoded by the 990) via this USB interface as well?

For the reasons Cliff states above I don't believe so. You need bit perfect output for DD and DTS.

I thought I came across an ASIO USB driver a while back but I'm not sure if it had full blown audio support as I beleive it was intended for MIDI.

For the reasons Cliff states above I don't believe so. You need bit perfect output for DD and DTS.

I thought I came across an ASIO USB driver a while back but I'm not sure if it had full blown audio support as I believe it was intended for MIDI.

The USB receiver accepts uncompressed PCM audio only and does not have pass-thru support for compressed formats like DD/DTS.