What happened to my room acoustics

FWIW, my room is about the same size as yours (19.5x13.5x9), but my set up is a tad different. I use the triangulated set up with my speakers 4' off the wall behind them, 9' apart, sharply toed in (axis crossing slightly in front of my head). My chair is about 4.5' off the back wall and placed to avoid a bass node at about 60hz. I have no special room treatments but I do have window coverings (heavy drapes), book cases, and soft furniture. 1st reflection points have been dealt with. My sound stage is excellent in that it allows very specific instrument placement over the entire wall behind the speakers. I measured my room, speakers etc endlessly (it seems) but ultimately it was tuned by ear.

The point of my saying all of that is to suggest that your chair might be too far back from the plane of your speakers which might just be a bit too close to the wall behind them and your low and midrange node may be the result of a room node which will change with moving your speakers forward. The HF dip could be the result of the off axis output of your speakers (but not so much reflections, I think).

BTW, FWIW, I know nothing special about your speakers. Also, for the most part, the measurement that really matter are those made with the mic at the listening position. Measuring your room or trying to duplicate the speakers spec's is really meaningless, interesting perhaps. Regarding sound stage, with my present system, the difference between my present system set up and one with the chair further back toward the rear wall was that the sense of depth diminished and there was some loss in specificity.  Those issues disappeared when I moved my chair forward (towards the speakers) to something closer to an equilateral triangle.

I agree with newbee.  Your chair is too far away.  You are hearing more of the room and less of the direct sound from the speakers.  Setup is critical.  Forget adhering to a specific formula.  They can be useful for sure but are not absolute.

In additional, the equal distance measurement from the front wall and side wall usually doesn’t produce the best balance.  I suggest you continue to experiment with your setup insofar as speaker position and listening position.

IME bass traps located in the front corners suck the life out of the sound with the exception of ASC tube traps which have variable reflectivity.  You might try what you have at the back corners if possible.

@newbee @rhljazz

Thank you both for the suggestions. Before I made the following adjustments for the speaker/listening positions, I tried treating the first reflection point, but the HF dip is still present with the same magnitude. I’m not sure why the dip shifts to a different frequency in my room as compared to the factory measurements though.

1. I separated the speakers a bit further, placing them at 9 ft apart, and pulled my listening position closer to 12.5 ft from the speakers. Previously, I had a long-to-short side ratio of 1.7, and now it’s 1.4. This adjustment seems to enhance the sense of soundstage depth somewhat, but it still doesn’t meet my expectations. Achieving the desired soundstage depth appears to be the most challenging aspect.

2. The S400 has a passive radiator in the back. I noticed that if I move the speaker further away from the front wall, the bass becomes noticeably less weighty.

3. I experimented with a sharp toe-in angle before because a few audiophiles here advocated it. It did improve the imaging, but the width of the soundstage suffered a bit. When the on-axis lines cross in front of you, it can sort of create a "crosstalk" effect. The crosstalk effect degrades the image but simultaneously enlarges the soundstage width. In both cases, I’ve sensed that soundstage width resulting from a sharp toe-in is typically confined within the boundaries of the speakers, whereas with a slight toe-in (approximately 6 degrees), I can achieve a soundstage wider than the speakers in my room.

4. I didn’t remove the bass trap because I found that the bass trap effectively addresses the standing wave around the corner, reduces unwanted excessive reverb, and, as such, improves the imaging.

Try a ratio of 1.2 and see if you hear improvement. Good luck ! 

The room measures fine to me. Many are much worse. People just don’t measure and they have no idea. 
 

two things stood out to me.
 

#1 the seating distance

with the speakers 8’ apart my max seating distance would be 10’ and for me I would try closer to 8’ as it would maximize soundstage width at the cost of center focus (personal taste). Then I would toe in until the center focus is back. The down side of what I described is you shrink the sweet spot but you get a better sweet spot imo. 
 

Being so far away also pulls in a lot more room reflections. Closer will be more direct sound. 
 

#2 the equal distance from the side wall to back wall from the driver face should be avoided. This will increase the null created by the wave length that has a 1/4 wave of 3’. Which is right around 90hz. It will also result in a peak but I am not sure where that peak will be, but higher like 180hz for example. I would move them closer to the back wall (I know this sounds wrong but try it) to avoid equal distances.

the low end roll off is just using small speakers without correction, it is what it is.