Correct, the 105D does not support 4K. You would have to get the 205 to support 4K.
In my opinion, the center channel is probably the most important speaker for tv/movie content. About 80% of the audio is targeted for the center channel. I think you should discard the idea of "rear surrounds" and use one of those speakers for a center channel. Even though the Oppo will decode and distribute audio without a center channel, you will have significant improvement in movie dialogue if you actually use a center channel speaker. I think the "rear surrounds" are a lot less important. You will still have "side surrounds" which gives you most of the surround effect anyways.
The XLR left/right outputs will send the same audio signal as the FR and FL RCA channels in the 7.1 surround output section. The XLR left/right is generated from a much better audio board, so it is better to use those for the front left/right speakers.
As far as the Stellar Gain, it is an excellent DAC and preamp, hugely better than the Oppo. You can actually connect it in between the Oppo and Levinson amp to make a "hybrid" system. Just connect the XLR outputs of the Oppo to the XLR inputs of the Steller Gain. Then connect XLR outputs of Stellar Gain to Levinson amp. You can actually configure the XLR inputs of the Steller Gain to be a HT input. This means that the input will be played at a fixed volume regardless of the volume knob on the Stellar Gain. This allows you to continue to use the digital volume in the Oppo for overall tv/movie sound.
At this point, I would connect digital coax output from the Oppo to a digital coax input on the Stellar Gain so that you can use the Oppo to play 2-channel CD or streaming such as Pandora, etc.
Then connect your PC to Stellar Gain using USB for other 2-channel audio.
You will not be able to get a 7.1 HT processor that will do 2-channel audio as good as the Stellar Gain. It's just not possible because the Stellar Gain has digital clocks that sync to exact audio formats (such as 44.1, 48, 96, etc.). A HT processor will also have a clock centered around 48khz, which is the primary format movie audio is produced in. Also, the audio stages in HT processors will not be as good as STellar Gain, unless you spend many thousands.
If movies are priority, you could go for a HT processor, but it will suffer in 2-channel audio. It would be better 7.1 sound than the Oppo, if you spend enough (Marantz 8802a, Krell Foundation, etc.).
As far as the sub, I would not even try to hook it up. The huge Levinson 333 and the B&W 801 will have enormous amount of bass strength. The sub will just "muddy the water".
