I don't think that diffusers have much effect in a very small room. What are your room's dimensions?
Ideally, I think that is important to place 1) bass traps in the corners, floor to ceiling, if possible, and 2) absorption panels behind the speakers, 3) behind the listener, 4) along the first and second reflection points on the walls, and 5) along the first reflection points on the ceiling.
Every bit of properly-placed absorption improves the sound. The more properly-placed absorption that is placed, the better the sound. In my room, after I installed bass traps in the corners and panels behind the listener and speakers and along the side walls, the biggest improvement came from absorption on the ceiling. I still need to deaden the corners more. The bass traps are 48" tall and are mounted in the corners mid-wall to ceiling, but when I clap my hands below the traps, I can still hear some ringing.
You can contact Sal @ acoustimac. Sal won't try to over-sell his acoustic treatments. If anything, he might underestimate what is needed, but he has a good product at a good price, and he knows audio.
Regarding the off-axis issue, the SongTower have good off-axis response, but I think the issue is the location of the listening position(s). There is a difference between treating a room for listening in specific locations and treating a room for listening from any location in the room. Regardless, the bass traps are a must, IMO. You might not be able to plan the whole room out at the same time, either. When I was getting started with my room treatments, I contacted Sal several times. At one point, he stated, "Isn't it funny how moving one panel can change the sound so much?" or something very similar. I didn't think it was funny at all, but it was quite amazing.