Adding two channels is probably the best solution if you want to keep the 7705 and not add another amp. I should point out that the QS8s are not recommended for ATMOS but I’d try it before changing them … most movies out there are not placing pin point sounds as much as placing more diffuse effects so the QS8s might be better most of the time especially in a low surround count system

I don’t know your room but two thoughts: 1) you might consider 7.1 before going ATMOS (more content will use it). 2) find room for a 2nd Anp as you may find that you will want to go to 7,1,2 in short order. Just my thoughts … neither is necessarily better then what your planning