Does the xbox have any type of output gain control? If the RR2150 plays louder using its tuner then the lower volume when running the xbox could be due to a gain issue. Possibly impedance matching as well. My thought is the RR2150 will not output its full rated power if the input is less than the design is looking for. Try connecting the xbox through the cd in on the receiver and see if that makes a difference. I do not know about the xbox but using my BDP-103 connected via the analog outputs the OPPO volume control will have an effect.

I would not be worried about the receiver. Had there been a major failure of the RR2150 it would have made itself known in a big way.

As long as we are methodical in the approach I am sure we will figure this out.