What happened to my room acoustics

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.

First of all, your listening position need to be 5 feet closer. Things are actually worse then they seem. The Y axis is in 20 dB increments. Most systems I have seen use 5 dB increments. This is a very shouty curve with the bass lagging 10 to 15 dB behind the midrange. From 200 Hz up things are actually quite good except for that dip at 12KHz. You need to measure each channel separately and compare. 

The ideal in room curve starting at 20 Hz which should be 5 to 10 dB above baseline sloping down and reaching baseline at 100 to 120 Hz, then flat out to 1 kHz then a steady slope downwards to 6 to 12 dB under baseline at 20 kHz. Bothe channels should mirror each other directly. I would like to see if that 12 kHz dip is in both channels. Any difference between the two channels will screw up your imaging. 

You need to deaden the front and side walls at the first reflection point which may calm your midrange and I would remove the bass traps. After that your best option is a digital preamp with EQ and room control. Subwoofers and digital bass management would give you much more control over the bass. 

Don't feel bad. I have measured many systems worse than this. I just measured a system with Magico S7 loudspeakers. The imaging was pitiful for speakers of that caliber. Low and behold one channel had a 10 dB dip centered on 300 Hz. He is ordering a DEQX Pre 4 which will fix the problem. This is like a photograph out of focus. You use digital EQ to focus the system and you can see the results with your measurement system.