In seeing your room I would lean towards room size as being the biggest challenge. Try moving your listening position closer to hear if it makes difference. The sound dispersion of your room may just be so wide that you are not getting the full effect of the array. Also the fact that the room opens up to another space is also going to affect the ability of the system to fill your room with sound. There are also a few reflective surfaces you may need to deal with. Even if the Onkyo only provides 65 clean watts per channel (it likely does better) you should get plenty of sound pressure in a reasonably sized room somewhere around 109 db spl which is very loud.