Howdy y'all,
I have found a double quad speaker base with additional side surround upper, 4.2.6 works best. The base level preserves the sterio image exactly as recorded, the rears (side surround) placed at 120 deg, handle both quad/multi channel perfectly. The height layer basically mirrors the base quad layer, with standard, directional speakers in front plus two pair of non directional (quad poles in my case) on the side to slightly in front and slightly behind (110-120 deg.).
I have fooled around with multiple speaker types and placements. Ditching the center is the single best improvement. I never found utility in the base layer Surround Rears, nothing has changed with new format. The second biggest improvement comes from using non directional speakers and adding upper surround rears. Since these are all synthesized output channels, they create a nice envelope, but still provide doppler shift and other sonic cues. And yes, two subs are better than one.
For those with space challenges, a double quad is all you really need for excellent DtsX/Atmos reproduction.
Full disclosure, I am a crazy person and spend way too much time with this stuff.