There really shouldn't be that much of a "sweet spot" for surround sound, as where the ribbons really only sound good IN the sweet spot, and roll off if you are out of it.
Surrounds should project the rear channels all over the back of the room... not shooting them directly at you.
Surround is the one time I could to recommend BOSE and not feel like a traitor
Personally I prefer Mirage Omnisat speakers for surrounds. They are small, and can project a BIG sound.