I had reached the conclusion the dips in the left channel midbass response starting at 180 Hz are occurring because the subwoofer doesn't have flat response to there. Measurements below show I was mostly correct, but as usual, the story is a bit more complicated. The panels do also have a bit of relative difference, with the right channel actually peaking at 180 Hz, cancelling some of the sub dip there. It does appear like the subs might have enough response to work more or less OK at 140 Hz with a steeper crossover like LR48, which I am pondering. Whereas if I move the crossover any lower, I might expose the panel cancellation at 110 Hz, which a 140 Hz crossover mostly but not entirely suppresses.
The measurements I made today are not necessarily the quietest. It is 106F outside and my air conditioner is running near its top speeds. I decided to leave it running, and just accept that as a given. The rough range of the subwoofer response should be apparent anyway. There are some slightly audible tonalities in the operation of the air conditioner, but they do not appear to introduce any notable spikes in a background noise measurement.
Here is the left channel sub (and supertweeter) without any EQ. Notice the large dip below 140 Hz, and especially at 180 Hz.
Here is the left sub with all the current EQ dialed in. Notably I have significantly suppressed but not eliminated the dip. This is with 5.5dB of boost at 174 Hz, and I wouldn't feel safe dialing much more in. The dip at 225 followed by a huge peak just above is very tricky to fix with EQ, though I could probably suppress the peak without making the dip too much lower.
The right channel subwoofer with EQ is similar at low frequencies but does not have the boost at 180 Hz, therefore 180 Hz is down but about 40% less than in the left channel without EQ. This seems to explain some but not all of the difference at 180 Hz the combined responses of both channels. Note also it has no peak at 225, only higher at 260.
Some of the remaining difference might be caused by the Acoustat response. Here is the left channel Acoustat response, with EQ, with the super tweeters turned off, but no crossovers (which does not make an easily visible difference at high frequencies). There is just a tiny dip at 180 Hz compared to the surrounding frequencies, though in the midst of the second highest plateau.
With the current panel EQ turned back on, there is very little difference there but you can tell the unmitigated peak at 4kHz right where Linkwitz calls for a dip. I think this could be the error that kept the Acoustat from becoming a market leader, instead of being displaced by Martin Logan and Sanders. This is somewhat tuneable with the HF level control, but not without making it less flat higher up. I keep the HF level control nearly perfectly centered. The Absolute Sound had the temerity to criticize the sound of the Acoustat 2+2, which could still be considered a feat of modern engineering, that perhaps lowlifes like me dare not criticize--it's far more amazing as a total package than anything I have come up with, and probably ever will. But, there may have been some truth in TAS whining. If Strickland could just have been persuaded to make it just a bit better sounding somehow, with a little nicer tuning of the response. The speaker sounds "coarse" like sandpaper playing pink noise without EQ, with EQ it sounds smooth despite the slighly more irregular looking response curve. The excess output with no EQ is centered right at the 6kHz "metallic" range. I concede my "smoothing" may not yet be perfected...and perhaps it's even "too smooth" sounding, once again proving you can't naively judge sound from the response curve picture.
Here is the right channel channel Acoustat with EQ. Notice there is actually a slight peak at 180 Hz, helping reduce the dip in the combined response on the right.
Here is the right channel Acoustat response with no EQ. Notice again the peaking from 4-8 kHz which I have imperfectly (but simply) reduced with EQ above.
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