There is one sticking point about ALL the new Onkyo receivers. If one has a speaker configuration containing Surround Back speakers, Onkyo receivers will output the Surround Channel information contained in Dolby 5.1 tracks from the Surround Back channels when using a Straight decode of the 5.1 signal.
Many do not mind the automatic up mixing of Dolby signals by the Dolby Surround up mixer in their 7.1 or more channels systems. But, some do not like up mixing 5.1 signals. If the 5.1 signal in play is Dolby, Onkyos will output the Surround channel information from the Surround Back speakers.
Outputting the Dolby 5.1 signal as multichannel PCM, if possible based on the device used for playback, would result in the Surround channel information being output from the Surround speakers.
The RZ70 and X4800H will each have their own quirks concerning DTS signals compared to older models of each brand. There will be some channel output issues with IMAX DTS signals as well.
If I was going to upgrade today and add two more subs to make a total of four, I’d probably get an X4800H to play with the Subwoofer “Directional” mode. I think it is still on sale for around $1,800. The X3800H is also on sale but I hate that front display.