when sitting in what will be the 'prime' position looking at the TV, is there a wall or a down-hang to the left that you can mount a speaker to?? if there isnt, then your best option would probably be to put them on the back wall extended out proportionally from the listening position. that seems like the only other option..
now, if you do have a wall or hang-down on the left(of listening position), then i would mount them around 2ft from the back wall, and around 6-7ft high. i think not putting them on the back wall would be the best way if you can swing it.. you might want to look into the FMB(full metal brackets) so you could mount to the ceiling on the left if you have to.
bigjohn
