Some kind of pressure dampening system is best for low frequencies- drywall on hat channel, mass loading, limp membrane for broad-band and Helmholtz Resonators can be tuned to offset problem frequencies. Lots of info about those and since you build the theater, you can control where they go to a greater extent than someone who is trying to treat a room that doesn't allow it easily. Don't forget that the ceiling is a good place to use treatments when the walls make it difficult.
I disagree about leaving the side walls alone- any time sound has a secondary path to the listener that's close to the SPL of the direct sound, it should be treated as hostile. Use 1ms/ft as your speed of sound and you'll be close enough for a small room. Measure the path length from the speaker and use a mirror to find the first reflection points- if you can see the speaker from the seating area, the wall should be made somewhat absorptive and the area to be treated is defined by the places where you can see the speakers. The fact that I didn't write 'speaker' as a singular means that you need to treat for left AND right speakers. I have a spot on each side that are causing reflections to be distracting when the SPL is high enough to make the reflected sound audible. One is coming from the side of a kitchen cabinet and the other is coming from a window- if I put something in front of those areas, the sound stops. In my case, the one to the right made dialog sound like it was coming from my cell phone, so I moved it and the sound remained.
Strong first reflections cause phantom sounds to occur, when the volume is above a certain level- below that, the direct sound will mask the reflected. It's not just the walls, either- ceilings and floors are additional areas that should be treated if you want to eliminate most or all of the problems.