I finally had a chance to experiment some more.
First off, the chairs definitely affect the center image. So thanks for mentioning that Kodiak. It wasn't a complete washout, maybe 3ish db; but I'll definitely pull the chairs back for critical listening from now on.
As for the null, I now think it's less of a center deficit than a boundary reinforcement problem. The extra bass is roughly 3 ft from each wall, which corresponds to the size of the front wall opening. So the speakers could be too close to the wall, or they could just be reflecting off the front wall (But moving them out as much as 3 feel didn't change things.) In any case, I think I'm good as-is for the low end; not that a sub wouldn't help in the grand scheme of things.
I also tried changing the toe-in again, and I think I had them over-toed. At least I'll try that way for a while and see if the difference remains a positive.
Finally I decided to leave the picture off-center. Too much change all at once can't be good.
