Incredible, I have been wondering about the same for a while, but could never find enough discussion about this. Usually it is about 5.1.2 to 5.1.4 (or 7.1.4) which is always deemed worth it due to how much more the extra pair of overhead speakers bring to the table. And just today, I was getting ready to ask the same question coming back from work, where I now see someone beat me to it
And the reason why I wonder, is because when Atmos was not a thing, I think that more often than not, the opinion was that 7.1 did not make that much of a difference vs 5.1. And this was my impression too. I went from 5.1. to 7.1 and did not get a sufficient bump in experience. I then downgraded back to 5.1 and never felt anything lacking.
But I am in the process of setting up for Atmos too these days, and am currently also debating the same question, does Atmos maybe make that extra pair or rear surrounds have more importance these days compared to before ?
And the reason I wonder is because, if I understand things correctly, before, the mixing used to be per channel, so the artist had to explicitly mix sounds for the rear channels, which maybe was not happening that often. But I keep hearing about Atmos being object / positional / 3D based, that is, the sound is not encoded for a specific speaker, but for a specific point in the 3D space / bubble. That does make me wonder if now just for that reason, it is as equally likely to have the rear surrounds producing sound as it may be for the side surrounds. If that is the case, maybe 7.1.4. vs 5.1.4 does make more sense and is more worthwhile these days, vs before Atmos, when we were comparing just 7.1 to 5.1. I could be very wrong about this though, so please correct me if so, it could be just a naive understanding of a noob (me).
But yeah, I keep wondering the same, is the extra pair of speakers, and the additional 2 channel amplifier worth it ? I did not think 7.1. was worth it before over 5.1, but if what I said above the difference in sound mixing with Atmos makes any sense, maybe 7.1.4 is indeed worth it these days (if your budget allows it).
I was looking at the Denon x4400 due to it being I think cheaper than the Yamaha 3070, but the thought of that extra amplifier taking space in the cabinet, potentially requiring additional work to turn it on every time (if trigger or autosensing does not end up working correctly like on various reports), the extra justification I need with my wife for the purchase of it even after purchasing a receiver, the thought of it making one pair of speakers maybe sound different than the other because the other will be powered by the AVR .. are all things that keep making me wonder whether the 7.1.4 will be worth it.
Maybe I should just keep saving, and wait until christmas, at which time maybe there will be some better deals on AVR that can power 11 channels with no need for an additional amp. Right now they are all more than my budget allows.