.This is obviously going to be a cut-n-fit proposition. I began by stapling up a pair of 2'x4' panels; one behind and above each speaker.
"What about first reflection points", says you. I want to carry forward the work I did on the front wall with the books and bookcase. There's about 1/3 of the 70" height which is reflective so "self", I said, "start behind the speakers".
Two sets of ears that are attached to some level heads told me recently that 'the treble is a little hot' on my system. All I can say is that perhaps my aging ears are a little 'cool' on top. Regardless, my first impression is much like when I put the speakers on sand bases: the bass is better defined and whatever series of events caused that also caused the mids to be significantly clearer and more. There are greater dynamics, as well!
I will put another 2'x4' panel on the left wall at the first reflection point, right next to the front door. To the right is the opening into the minor leg of the "El"-shaped room. I'll have to build a frame to see if a panel would help there.

I've always believed in the efficacy of sound-conditioning. It's simply that I was apathetic about it. No more