The Bonello chart shows modal DISTRIBUTION - Bonello's theory was that you need an even distribution of harmonics in a room to keep from coloring the sound. If you click on the Bonello tab, you get a large enough view to tell that the blue bars are as tall or taller than the red ones. This is good...
I ran your dimensions, and MODESv2 didn't show up any problems. Then I ran them in roomtune, and noticed a slight bunching up around 209 hZ - H3, W4, L5 are within 2 hZ of each other. Since these are 3rd, 4th and 5th harmonics they shouldn't be as noticeable as primary modal freq's, but still may need some treatment.
Using Auralex to treat that low a freq would require the 4" stuff, and you might be better off costwise building one of John's absorbers, tuned to about 200 hZ. Or, if there's room (might be kinda tight in that room), maybe just try throwing in a big(medium), fat couch behind the mix position. The back wall would catch 2 of the 3 axial modes (height and width), which would probably be enough.
The other thing is, with that small a room there isn't enough "travel time" for rear wall reflections to be just diffused, they will still muddy the stereo image at the listening position. I'd expect you to need broadband absorption on the rear wall, which isn't cheap to buy (4" Auralex, for example) YOu haven't mentioned just how far you can go with modifications - if you're not allowed to pick up a hammer, you might be stuck with the expensive way or building PORTABLE absorbers... Steve
