If you go ATMOS you are likely looking to add an amp anyhow. Most AVRs only drive max 9 channels right now.
What you are referring to is going seperates vs an all in one receiver. There are pluses and minuses to both and strong opinions for either being better.
I chose a hybrid approach, as many do, and will run my front channels on outboard amps and let the AVR run the rest. Most of the soundtrack (over 90% according to hometheater geeks podcasts) comes from the front 3 speakers. I doubt this will change much, even with ATMOS, as the sound must still be anchored to the picture.
An AVR is the "brain" of the system while an outboard amp is the "brawn". I much prefer the theoretical "capabilities" of the hybrid approach..... Theoretical because mine are still in the box.
Putty applied to electrical boxes today.