Here are my thoughts and setup details for using a second AVR to power the Atmos heights.
I am running a Marantz SR7012 with Auro 3D speaker setup so I have 4 heights and a Top Surround (Voice Of God). Now the 7012 will power it as 5.1.4 but I needed an amp to power the VOG speaker as the output to that is supplied by the 2nd subwoofer pre-out. So rather than buy a mono amp I decided to use a spare Denon AVR1910 receiver. I was also unhappy with the 3d surround effects I was getting as it didn't seem to be much effect at all.
So I plugged in and wired up the Denon to power the heights and VOG and what an instant improvement - but that might just have been because the Denon was powering the heights a lot more than the beds without any setup.
So I proceeded to setup the Denon and was wondering about speaker levels, distances, master volume, Audyssey etc and after thinking about it realised that none of those matter because once you run Audyssey (or whatever you have) on the master receiver (the 7012) it will compensate for whatever the slave receiver is doing anyway.
Lets say the speakers are physically 3m from the MLP but front height left is set to 2m and 0db and the front height right is only 1m and -5db on the slave. When Audyssey is run on the master it will measure the difference in delay on these 2 speakers and will (should?) set the levels and distances to compensate for processing by both receivers.
But just to make it easier and more logical I set the Master Volume to 0, Speaker distance to 3.6m and levels to 0, turned all processing off and then reran Audyssey setup on the 7012. The new setup was as close as dammit exactly identical to the old (before I added the Denon). Speaker distances WERE exactly the same and the levels changed on the heights but they were all the same differences as before - all were increased by 5db. In light of that I increased the set volume on the slave receiver by 5db and reduced the level of the heights by 5db just to bring them back in line with the rest of the system.
It now sounds like it should and I am pretty sure there seems to be more detail and clarity overall - at least it seems to sound like a huge improvement to my ears - and the 3D effect is as it should be
And an added benefit f this setup is that if you want to change the amount of height effects, either more or less, then it is just a simple matter of adjusting the volume on the slave receiver.
I haven't had a chance to max it out volume wise yet, but I suspect it will play louder and better than before.