First, can you please tell us a little more about the room and the system they will be used in to better help us with recommendations?
For a 5.1 setup, the recommendations are to place the surrounds on the side walls, at around head-level or just a tiny bit above (so listeners on the same row do not occlude the others) and slightly behind the row of listeners, at around 110 degrees from the line to the center speaker.
Here is the diagram from the
Dolby site for a 5.1 layout
Now, the FX2 has tweeters aimed forwards and backwards, so I'd increase the angle to the listeners to around 120, but ensure the bounce angles of the rearward tweeters combine at the MLP.
If the location is too close to the rear wall, then you'll need to either dampen the rear-facing tweeters output (or disable it by modding it) or switch to a monopole speaker model, such as a 35XTi.
The design of the FX2 reflects old-school surround practice from back in the days when the surround channels were not discreet, and a diffuse soundfield was desirable for the rear.
In today's discrete 5.1, and especially for immersive audio (Atmos), the rears need to be directional and have smooth dispersion.