My setup uses a pair of Sequels as the rears, and I like the results a large line array gives as a rear speaker. You will want to do similar to what I did and run an abffussor behind them to help spread out the sound and reduce dipole cancellation effects.
Here is a pic of the rear of my system where you can see the RealTraps Abffusors (grey side/white vertical diffusor) placed behind each sequel. I use 3D diffusion in the middle (4 RPG Skyline units).
The woofer boxes are the
MidBass Modules that go behind the MLP and spouse's seats (removed for this pic).
The coffin-sized box between the Sequels is my rear 18" sub.
The Theos is not as tall as the older Gen1 speakers, so I'd recommend placing them on a 5 or 6" tall base, and then raising the rear feet so the panel is vertical.
Just like the front, an ESL wants some space behind and to the sides. See my room layout for an example of what it takes.
If you are forced to reduce the space between the rears and the rear wall, then a MiniTrap HF would be preferred, as that will cut the comb-filtering from being too close to the wall. Ambiance restoration can be had with some small squares of angled solid surface materials to bounce away some of the signal to the sides.
