Yes I agree with Hexspa - it is worth experimenting by moving your speakers around. Backwards, forwards, wider, narrower and up and down. Sometimes quite a small adjustment can make a difference to a frequency peak or notch in your room. This is not always the solution.
All rooms have peaks and nulls. The best you can do is aim to make them as small as possible using bass traps and by varying your speaker placement and listening position. If you can get the span between the highest peak and deepest null to within a 10 dB window, you're doing very well!
I agree with Hexspa on all counts. And as for Hex's observation that the width is responsible for the 85Hz notch, bass traps along the side walls/ceiling (and floor) should help if you don't have them there already. If you do, add 4" (or thicker) absorbers spaced 4" off the side walls. If you have all that in place already, I guess you're done.
I couldn't find in these forums how to do a waterfall, but i tried something. Is this better ?
Now i feel like a completely novice.
Targets vary but the one I use is 20dB decay within 150ms above 40Hz as suggested here.
You can also use the decay graph but sliding the values around on your own will help you.
To qualify the last statement: by values I mean the lowest dB value. Since you're aiming for 20dB within 150ms, and every frequency has it's own max value, you're going to have to find those ranges which are greater than your tolerance. This is all pertaining to the waterfall graph. If you use the spectral decay graph then this is easier to see. Unfortunately REW doesn't have 150ms as an exact display value in that graph's settings but you can get close by averaging the two closest values. Again, that's just one target by one guy but it seems as good as any. In my experience of measuring my room and seeing others' it's a fair target which divides rooms which I consider to have been well-treated and those that in which compromises have been made or faults have manifested.
Lastly, if you canter the waterfall a few more degrees where the low end is closer to the viewer it's a tad easier to read.
Looks good. But since Hexspa did the math and identified the width as the suspected culprit for the 85Hz notch, if you want to improve it, spacing the side wall absorbers 4 to 6" off the wall will help. Those corner upper traps are too small to do much but if you leave them up and place 2'x4' x4" or 6" in the spaces between windows, doors and wherever they fit, you may attack the notch. If measurements confirm we're on the right track, you might want to add as much as you have room for.
Yes, I again agree with Hexspa. Since you are handy with using REW, your can track your progress. Since LF tend to be focused into the corners, that's a good place to add more treatment. To get the most effective corners bass traps, make sure you use FRK or paper/plastic facing the room. As I mentioned in an earlier post, spacing the wall panels off the wall may be the easiest since the panels are already built. If you measure with REW after you space them, you'll see how much (or little) you have gained.
One idea is to use a couple 2x4's mounted vertically (They are only 3 1/2 inches so that may be enough or you could attach a 1x2 furring strip to the front of it ?)
Anyway, if you go with the 2x4 idea, you'll want to bore a holes a little larger than the screwhead edgewise into the 3 1/2 about 2 3/4 deep so you don't need so long of a screw to mount the 2x4. With the 2x4's vertically attached the wall, you attach your panel to that.
Something else that just popped into my head is build trusses out of 1x2's instead of 2x4's; More labor but cheaper materials and more acoustically transparent.
If it were me I would first try this out without installing the panels i.e. I would first try placing the extra wall panels vertically on chairs spaced 4" out from the walls on both sides of the room. Once I got the effect I was looking for I would then consider installing them on the walls. I say this from experience of trying to rid small to medium sized rooms of lower mid nulls and peaks - it's very very difficult.