Right speakers for big room limited to small room placement options


My listening room is 26' x 26' with a vaulted ceiling rising to a peak of 20'.  Total area volume is 11,000 cu. ft.  I am limited in positioning speakers: must be on either side of a fireplace located dead center in the "back" wall.   L\R Speaker separation is 84" maximum, center line to center line.  The front edge of the speaker can be no more than 26" from the "back" wall.  Using Cary SI-300.2d, with Cary streamer DMS-550 for electronics.  Interested in anyone's thoughts and suggestions.  Using satellite sub(s) is doable but not first choice. 
billbouc