I believe this is incorrect. USB is a digital communication interface, and as such it isn't capable of carrying analog audio. The RR2150's USB module contains a DAC as well as the other electronics that allow it to act as a sound card - the PC's sound card is not in the signal path, and the USB module itself serves as a sound card (one might even say that the entire RR2150 becomes a giant sound card when used in this manner). I know that the Model 990 works this way because I've tested it. The RR2150 has to work the same way.

This is typically true. There are of course going to be exceptions, but as a general rule stereo receivers are analog only. Surround receivers must have DAC's to function, since the surround processing is all performed in the digital domain (although I think Fosgate built a purely analog multichannel pre-amplifier a few years ago).

Yes, the 990 can apply processing and do D/A conversion for the USB input. I think the root of Carlos's question, though, is whether Windows mucks around with the audio data prior to handing it off to the 990 or RR2150. On that point, I'm just not sure - I certainly would like to be able to insure that the signal path was as straight as possible within the PC if I were using my USB input for much, but I also have spent enough time with Windows to know that it can hide a few bumps in the road when it comes to stuff like this.