I thought I read you weren't supposed to angle them downward? Mine are about 6 feet off the ground but not angled down, slightly toed in though. I have tried lots of different things and this sound best.
I'd agree much of it is personal preference. Mine are just over 6' heigh, 18" behind, and angled down and forward toward the LP. I've experimented a lot, and IME and in my room, that seems to tie them to the front speakers and creates very cohesive surround bubble. Imaging from front to rear and side to side is excellent. I even get good imaging behind the LP in 5ch playback (vs7ch). I look at surround placement just like L/R placement. Modern soundtracks are full of directional discrete sounds so IMO, surrounds should be set up accordingly. Just saw pogre posted...... let's go see!
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