despite what has been posted here, you most certainly utilize a 5.1.2 setup with your current receiver for ATMOS/DTS:X. also, you and others have to understand in the real world, we have to make our given situation work. if your current "surround" channels are at ear level and not higher you can as you mentioned mount 2 channels to the back ceiling or as close to the ceiling/wall juncture (since you said ceiling mounting near the middle of the room isn't possible) and give them a 45 degree tilt or as close of an aim as possible towards the MLP and set them up as "Top Rear"
and you will be fine.
everyone won't have absolute "dolby specs" (which i consider more recommendations) in their installation so, you have to tailor the system to your room as close as you can to the "dolby specs" it should still sound good. i have a 5.1.2 in my bedroom with the speakers mounted onto the sheetrock ceiling and they are "top middle" and my surrounds are at ear level in the back of corners of my room angled between the two walls. works perfectly.
sometimes, you've got to make what you have work. that is my position/opinion.
best of luck, please keep us posted and please do post some pictures.
i just checked your manual and you don't have a Top Rear option, only Top Front or Top Middle.
so, depending on the location of your surrounds, again, assuming they are mounted at ear level, you could mount the height speakers at the same wall/ceiling juncture aimed down towards the MLP and assign them as Top Middle or Top Front in relation to and distance from/to/between your current surrounds and front. that should give you the best effects possible.
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Last edited by solarrdadd; 02-18-2020 at 12:02 PM.