It depends on whether the mixer has assigned whatever elements that appear in the side speakers to a stereo object or part of the bed. If they're part of the bed, even with an Atmos AVR that information will be sent to the rear speakers in 5.1.x. A stereo object will render more accurately between the front and rear speakers post Atmos decode, but with an older AVR that stuff still ends up entirely in the back.
Thanks for that explanation, which might account for why some Atmos mixes sound especially rear-heavy on my system, even though most sound well-balanced (if they're done well in the first place).
If I'm understanding correctly, you're saying that if an Atmos mix was done using a 7.1 bed instead of a 5.1 bed, the side content will go to the rear speakers of a 5.1 system regardless of the AVR. But when you say that a side object on top of a 5.1 bed renders between the fronts and rears, is that just for an Atmos AVR, or for any AVR?
I'm not sure it matters, and I don't want to derail this thread too far off-topic, but I have an old setup except for the ATV box and a recent LG TV. My decades-old Yamaha AVR doesn't even have HDMI inputs. The ATV HDMI output goes into the TV; the TV feeds the AVR through a digital optical (toslink) cable. I get 5.1 Dolby Digital through this connection, from video streaming services and cable channels funneled through the TV, and Atmos rendered into 5.1 from Apple Music on the ATV. (I also have an old Oppo BDP-95 with analog inputs into the AVR for high-def 5.1 and stereo, but that's not relevant here.) As old-tech / low-tech as this may sound, it works fine for my needs and I think it generally sounds really good. But maybe that's just because I don't know what I'm missing?
In any case, I certainly appreciate and accept all of the feedback I've received here that these Zappa albums sound great on a true Atmos system. I will play around with this more on my end, listen to some other Atmos mixes I've found in the past to be rear-heavy, and see what I can come up with. And maybe I'll spring for the Waka/Wazoo box set to get the dedicated 5.1 mixes.