That must be it, similar responses but different enough so they can still be filled by each other. Then almost nearfield considering their size, and depending of your listening levels with that headroom the distortion is probably still good enough.
See below R and L and both MMM (no EQ) and R and L and both Sweep with EQ. While sweeps have more information I am never sure exactly what I am looking at below Schroeder especially for just one position as you can't gate them effectively. The right channel has a worse dip around 100 Hz which I believe is the combination of the corner and the ceiling height change close to the right speaker. It has been suggested that stereo corrections are better for imaging so I will try that. If you see anything interesting I welcome your comments.
I made individual channel EQ instead of one EQ for both channels. It ended up taking another 2-3 dB of headroom, not surprising with the extra large dip at 100 Hz on the right channel. I tired some listening tests and I can't reliably tell the stereo and mono EQ correction apart. I am sure it depends on many variables with the recording probably being the biggest. For now I am using the stereo EQ as it doesn't seem to do any harm and I still have plenty of headroom.
I played LF tones (LF pink noise and pure LF tones) and then walked around the room listening for changes. In the area around the LP and the center of the room I didn't notice much but near the sidewalls and especially near the rear wall there were some easily audible large dips and peaks. The other thing I have noticed is that speaker and LP height is just as important as front to back / side to side speaker location and LP location.
Back when I was doing my RoomEQ I found that measuring each speaker individually and then doing per speaker RoomEQ did not work as well as just sweeping both speakers at the same time and then doing RoomEQ on that. I remember getting an easier to achieve flat in room response by just treating it as a mono system.
For me at least mono EQ isn't as effective as stereo below 200Hz. Even though mono FR of 2 speakers broadly matches individual FRs, I'm guessing there are enough differences to require seperate EQing.
It is important but for most cases, the listening height is fixed (in my case, it's 120cm sitting down) and conventional wisdom requires the tweeter/HF driver height at or above/below the listener's ears no more than 15 degrees for 2-way speakers. In addition, ceiling treatment is tough (some rooms are simply unsuitable) and can be hard on the eyes. Treating the opposite surface, the floor, is doable but you run into the same problems.
I was able to fill the gaps in the frequency response by mono EQ'ing more completely than I could using per channel RoomEQ. I'm trying to remember how I did it. I did roomEQ on each channel and then I did a frequency response sweep when both speakers were active and the response was no longer flat so then I did RoomEQ on both channels - so there was like two layers of RoomEQ and it still didn't fill the gaps in the frequency response as well as simple mono EQ'ing of both speakers together. I don't really remember AB'ing the sound quality between the two versions though, I just decided to go with the result that achieved the flattest frequency response. If you're able to fill frequency response gaps better by doing per channel RoomEQ then fair enough for your situation.
I agree ears should be aligned with tweeter, what I am talking about is the height of the LF driver. Many subs have the driver very close to the floor and some have them elevated from the floor and the mains LF driver will be at a different height from the floor depending on it's configuration. When I measure LF room modes I get quite different results depending on how high off the floor the LF driver is. This can be as helpful or harmful as moving the subs / mains / LP left and right and back and forth.
After comments I received about possible imaging issues I tried a "stereo" correction and compared it to a "mono" correction. The "mono" correction gave a smoother measured response and consumed 2-3 dB less headroom. The main issues is around 100 Hz and I could not hear any difference in the image or anything else. I do notice that audibility of LF changes is very material dependent so my quick listening check is not definitive. It does make sense that a "mono" correction will usually result in a smoother response and consume less headroom at the expense of frequency dependent channel balance the audibility of which is hard to predict.
Dirac and MSO will both boost nulls though, especially in multi-sub modes. MSO's EQ configuration has eliminated several nulls in my room for many years now. So I think you maybe just had a misconception about what those other EQ techniques do.
In the 70s Allison and subsequently Acoustic Research discovered that placing woofers close to the floor and the walls can lead to smoother in-room bass. The AR9/90 is the epitome of this design choice, with 2 10" woofers to the sides crossed over to an 8" midrange driver at 200Hz. When you set these speakers up in accordance with the manual, they don't have huge peaks or dips below 300Hz like most speakers (at least according to AR's own measurements).
I see, so stereo correction wasn't necessarily better for you then. (And not better re measurements).
Yes, since the mono correction measured better (very subtle) and I could not hear any difference and since the mono correction required less head room that is what I am using.... for now at least
