My HT room has beams like yours - for a moment I thought you were living in my house.

I have QS8s and they are EXTREMELY forgiving on placement. Mine are on the back wall like yours but a foot or so lower as bridgeman suggested. I might also move them a foot or so away from the side wall but that might not be necessary. What is necessary is to calibrate your system when you are done. I ran mine uncalibrated for a while and believe me the difference is significant. Good luck.