Philippe, by "90 degree" you apparently mean that the side surrounds will be directly to the side of the listening position rather than slightly farther back. While in a 5.1 setup, as you mention, it's better to have them 2-3' back, directly to the side is still very good, especially with the QSs.

So don't be concerned, although if you could move your seat a couple feet forward, it would be interesting to see if you prefer it that way.

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Enjoy the music, not the equipment.