I improved the response of the right channel from the first spectrum to the second one shown by moving the low pass crossover frequency for the subwoofer from around 160Hz down to 121Hz. The high pass crossover on the electrostatic panels remained the same 121Hz in both cases. Previously I crossed over the subwoofers higher than the panels deliberately in order to fill in what I perceived as a low point in response between 120Hz and 200Hz. In the overall scheme of things, that depression (still visible in second spectrum) doesn't seem as important as the 80-150hz peak that arises from this crude attempt to reduce it, and the bass sounds cleaner (less resonant) with the 121 Hz low pass. Been thinking about pushing crossover even lower, but I don't like to put too much of the low frequency spectrum into the panels as their low frequency transformers can melt, so 121 is about as low as I want to high pass the panels for reliability reasons.
The performance was also improved from previous spectra by increasing bass generally in both channels (through the crossover) and individually in the right channel by setting the volume control on the sub. (I can't remember if these changes got in the first spectrum above, possibly not, though it actually looks like there is some improvement there.)
Also, the supertweeter level was reduced about 5dB, that affects pretty much only the two highest frequency bars, where it is sometimes hard to see any change. The supertweeter is not intended to be very audible, instead just at the margins of audibility, and filling in the upper frequencies which we don't think we hear but might have perceptible effects anyway. Normally, the supertweeters help remove the bandwidth limited "plasticy" sound from the Acoustats; recently they may have been adding some undesireable aggressive steeliness. On other analyzers, it often appears like the supertweeter is improving the response of the highest frequency bar by making it flush rather than lower than the preceding bar, thus extending the high frequency response and making it less peaky.
As the last paragraph makes clear, though spectra look impressive and "scientific" when presented, you have to remember that each measurement is but an imperfect sample of the processes involved, and that better measurements can often put things into a rather different light. Visible and rationally analyzable spectra greatly facilitate progress in audio design, but should never be considered the end goal. The end goal is better sound, and the spectrum is only a tool that helps modify a system towards that goal.
I would classify the frequency response in the second graph as "pretty good", and much better than the average system, but still in need of work for the ultimate level of performance (what I was claiming to provide previously...now again a work in progress since my discovery of low bass in a previous post).
The suckout in the 4K-6K region is troubling, probably deserves more investigation, though most likely is a speaker limitation that can't be cured but can be corrected by the Room Correction System.