Last night I surprised myself by discovering that last years listening position was 33% from back wall, and new closer position is 38.4% from back wall. (I had thought it was actually in the front side of room!). The TV and stereo stuff in the front actually extends out almost 5 ft max (though behind the speakers is 3 ft min because of angling).
I tend to like frequency balance in old position better, but imaging greatly improves with new position.
I've been so preoccupied with room response that I overlooked the obvious. What I need to do is change the speaker toe for the new position. The too-bright frequency balance is probably being caused by being very close to axial now. Before I may have been a few degrees off axial, enough to make about 3dB difference in the highs above 5K or so according to freestanding measurement I did this morning.
It's so obvious now that Jim Strickland designed the speakers to be best slightly off axis. He personally recommended slight toe in from listening axis, toe out might work as well. And most likely speaker was designed this way (1) greater flexibility and (2) to compensate for HF beaming and retain highs in ambient response (a typical speaker design tradeoff). The membrane is physically flat, has no tweeter, 9 inches wide, serious beaming is inevitable starting at something like 2khz.
Alternatively, I could turn down HF balance control. Both speakers seem to have identical balance, despite one having NOS interface replacement two years ago. So HF balance control(s) (inside interface and not easy to change) must be at factory setting.
My needs are so specialized in this tightly packed room I may indeed end up having to make stuff. I'm wondering if bookcases could be re-designed to turn them into absorbers. The backs are some kind of cardboard-like material usually.
Most manufacturers don't say what membranes they use, but one says they use 1lb/sqft barium loaded plastic. Then behind that is typically 3lb/sqft fiberglass with some sort of air gap on either side.
WRT the higher resonances, like 215/217, since those could be affected somewhat by positioning. They have midroom peaks as well as midroom troughs. Though, within working range I am now, perhaps not much difference is available.