Provided you have enough room to place the panels away from the back wall, the backwards radiation is a feature, not a problem with such speakers. The out of phase signal cancels to the side a bit, given that bass waves wrap around "small" objects like a speaker. The rest of that back wave bounces off the walls and gives the impression of a bigger soundfield. Yes, there will be some cancellation and reinforcement from soundwaves interacting, but, that is the case with front-firing speakers too, once the sound starts bouncing off all the various surfaces in a room. If the panel is at least five feet from the back wall, its reflection off the back wall will be delayed enough that your ear/brain will separate it from the direct sound of the front wave and it will be perceived as a reverberation and will not mess up what you hear from the direct sound.
Don't bother with trying to supress the back wave. Make the effort to place the speaker where it will sound its best which might mean trying to delay the reflection back to your listening position for as long as possible (meaning keeping the speaker as far from the wall behind it as possible and perhaps having enough randomness in the surfaces behind the speaker (bookshelves, potted plants, wall hangings, etc.).