I think I explained it pretty clearly before, why don't you re-read my last post until it makes total sense to you.
Here's my theory, which is why I asked if you were hearing anything front the center when it said "Dolby Digital":
When you are playing from the Xbox, it is outputting at DD 5.1 and just sending blank channels for everything but the fronts. So you are hearing the music in stereo, as the receiver is "tricked" into thinking it's getting multichannel. So even though it says Dolby Digital and 5.1 boxes are lit up on the left, the receiver is only playing the music in stereo (nothing comes out of the center) because of the blank channels.
When you stream, it outputs the raw 2-channel PCM, and the receiver locks into Pro Logic mode and you are getting a matrixed version of the music, with part of it diverted to the center channel.
I believe that either:
1. your center channel is inferior to your fronts and so the music sounds worse when matrixed with pro logic, or
2. there is a simple volume mismatch between the two outputs
Or, some combination of both. I bet if you put it into "Stereo" mode instead of DPLII Music when you are getting the 2-channel from streaming, they would sound much more similar. And if you matched the volumes, they would probably be identical.
The key thing you need to understand is that IT IS STEREO TWO CHANNEL either way. Your Xbox is just doing something where it tricks the receiver into thinking it gets dolby digital, there is no such thing as the "Dolby Digital mode" for stereo music. The only reason the receiver says "Dolby Digital" is that it's being tricked into doing so.
When it gets the correct two-channel signal from Media Center, it defaults to Pro Logic mode which is an artificial matrix mode. I bet if you turned it off and just put the receiver in STEREO mode it would sound normal.
Anyway, you don't even have a xbox right now, so don't sweat it. You are obsessing about a non-problem. Just wait until you get the coffin back and you can experiment a little and verify what I'm saying, until then don't worry about it.