@soix @yoyoyaya I did more experimentation with placement today, like dramatically pulling them away from the back wall, and still no dice. Then on a whim, I also tried something new. The Electra 926 are designed for biwire, with the lower posts driving the woofers and the upper posts driving the tweeters and mids. I tried removing the in-line, high-pass filters between the speakers and the amp, and then the jumpers at the back of the speakers, and only driving the upper posts, then disabling the low-pass filter entirely on the subwoofer to fill the gap left by the unpowered woofers in the main loudspeakers. I managed to get response flat through the crossover band, but the quality of the upper bass in the 200Hz range coming out of my sub is complete poo compared to the woofers in the Electras (yeah, no surprise, right?), so I've once against reverted it back. I felt an improvement in soundstage without those high-pass filters in place, so either the filters are creating a bit of a veil, and/or the amp only driving the mids and tweeters gives it less work, enhancing quality there, but the completely muddy upper bass in that configuration just kind of killed the entire vibe.
The experiment continues...
-Ed