yes you can do that.

If you have also a "digital coax" input, you may try to obtain a cable box that has that output, and then you can get digital sound from all/most of your components. However, Im not sure if the xbox and/or your receiver decodes digital surround formats like DD 5.1, DTS, etc.

Another thing I would experiment with is in finding if there is any difference in PQ by running y-pb-pr directly to the tv as opposed to the receiver. You very well might, or might not. Less convenience, but its not too hard to change TV's inputs via remote. My 2 cents for now, gl, J.