You have a great system! I would love to hear that combo! How do you like the highs? The salk's ribbons sound incredible with AVA tube electronics, not quite as perfect even with Franks Van Alstine's great ultra550 amp. With all the crap written about classD treble, I would love to hear your opinion using them on such great treble speakers.
Your freq plot looks pretty damn good, BUT there may be some fiction there... You obviously have the plot averaged by 1/3 octaves. To really see what's going on, you need to plot the results without any smoothing at all, just the bare naked data. High Q nulls and peaks can be as little as 2 Hz apart where they would average each other out with 1/3 octave smoothing and hiding the problem. You also might wanna sweep from 20Hz through the entire bass range all the way up to 300Hz, as mid bass goes up that far and problems in those higher bass freqs are more noticeable because we hear mid bass better than low bass. You can also sweep all the way up to 20kHz to see how much comb filtering you have in the mids/highs. As you add room treatments, it is nice to have a reference of the entire freq band so you can look back and see just how bad it used to be! Helps to make you believe all the money spent was worth it! haha
I don't know if sweeping at a lower volume would make a difference. I think the room behaves the same at all volumes at least in theory. It makes sense to me to measure at a volume setting close to the level at which you listen to music. I measure at ~75dB, your looks more like 90 average. See if measurement at 75 looks any different.
Try making several sweeps with the speakers moving away from the front wall in increments of a foot or even 6 inches! You will see a big change in the freq response with this movement, as well as notice big changes in the sound. You will probably prefer the sound with the speakers out from the wall as far as you can live with. You may get advice about putting absorption behind the speakers to reduce SBIR. Try it with a pillow or something free before you spend any money on that
It can kill the ambience and flattens the soundstage in a hifi situation. That technique is better suited to a pro audio mixing, mastering situation where ultra clarity needed at expense of musical enjoyment. The room adds a lot to the enjoyment of your system, as long as it is tamed. Killing all reflection sucks out the life.
Use some really good recordings of classical music or acoustically recorded music with instruments whose sonic timber you know well. That way you can really determine whether it sounds natural or if there are problems with tonality caused by the room. A freq response plot only measures amplitude, it does not tell you about the quality of the sound, tonality, etc although it is a good start if it looks relatively flat. If it sounds shouty, harsh or just "worse" as you turn up the volume, it may be your corners, as Nathan suggests. I think that the horn amplification effect of the corners makes the phase distortion from the corners more noticeable with increasing volume.
If when you plot your data without any smoothing you find there are no nulls (dips) and just that mild peak at 40Hz you are sitting pretty. I think you will find the real data much less pretty. Too bad smoothing the curve in software doesn't change the room sound too! TacT does it, $$$
Enjoy that great system koiman!
Rich
