In a large open room like this, surround speakers can be challenging to do and ceiling speakers are an option, although they are not a great option. You say the rear left and right open, do you have any side walls near your listening area? How about the back wall? It sounds like you are trying to configure 7.1. If you can get away with aestheically speaker stands always work well in open room. Aa for the atmos, DO NOT do in ceiling surrouds and atmos enabled speakers... Your sorround channels will be coming from the ceiling but you ceiling channels will be coming from the floor and reflected off the ceiling... If you have to do yiur surrounds in ceiling, cest la vi it will woek better then nothing, however if thats the case, then just forego the atmos.
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