If you are digitally connecting the streamer to the Denon, you do not need the Ultra (unless it is the screen you like). When connected digitally, you are using the DAC in the Denon. Also, if you are doing any DSP functions like bass management or room correction, any signal going to Denon gets digitized for the DSP function, and then also ends up getting processed by the Denon's DAC.

Personally, I would steer you towards a Rythmik sub. I would also wait on the amp as you may not feel you need it.

What is the cause of the indecision on the center speaker?

I hope you can get the ELX's in white and it isn't cost prohibitive.