9' or 3m is a good distance from the rear wall for some diffusion, rather than absorption or you could try a combo diffuser/absorber.
For my tastes, since your in the nearfield and near the front wall, I'd treat the front wall with some 4" panels, especially behind and in between the 2 speakers. This will treat the SBIR you will likely be getting off the front wall (probably in the 200hz region).
The cool thing about your setup is if you needed to, you could run something like a tuned turbo trap or something like that (that's what GIK calls them) under the sidewall traps and it would fit nicely. Those would deal with some of the lowest frequency issues, if you have them.
Otherwise, I'd say it's probably at the point where you're not going to get a lot of benefits from covering the remaining areas.
Was definitely thinking of adding some absorption behind the speakers, will just have to make sure the GFR and depth of the material will help reach low enough to help with the SBIR. Think I did some calculations once, and with a speaker as deep as the KH150, the perfect absorber was only just hitting around 0.6 absorption coefficient at the SBIR frequency. Moving the speaker back would raise the frequency and make it easier to absorb, but then the absorber won't be as effective down low due to being thinner lol. Must be better than nothing though, as people seem to have good results with them.
I was thinking down the track playing around with dual KH750 subwoofers located under the side panels (around middle of wall) but I currently have a single unused PC4000 SVS subwoofer that might do the trick. If a single subwoofer ends up working up the back of the room, using that space for membrane style traps could be a good way to use that space. Since the room is tiny every inch counts!
This whole project has been a real eye/ear opener, I prefer listening to my crappy 4" speakers in here more than my big costly system out in the living area. Renting has always made experimenting with this kind of stuff difficult.
Thanks for having a look over my stuff Ozzy