The thing about the sound... Well there's a couple things.
Just for being in there and talking, it's pleasant. The low noise floor makes it easy to talk, and having at least one fairly dead end keeps things from being too flutter echo-y. There is still a flutter echo, but it's pretty mild, and should be basically gone once carpet is in. It could be that the left and right walls persist in making a flutter, but it doesn't feel like it. I guess we'll see in a month or so.
I do a good deal of casual listening while I'm working in there. I have the L and R hooked up to a SONOS connect:AMP for now. They're toed in moderately: enough for approximately? an equilateral triangle. Stereo imaging is strong and stable, as you would expect from SEOS designs. When listening, I don't notice anything weird; no flutter, no crazy resonance. I do notice that I need subs, but that's to be expected; F3 is right around 80 Hz.
Overall, I'm optimistic that I'll be very happy with the sound with a couple additions. I think I'll build a couple hybrid slat diffusers (1D horizontal diffusion) for the rear walls with a couple inches of duct liner behind the slats and probably be done. If a flutter echo persists, I'll probably find some lateral wall space for more hybrid devices. I expect to leave the first lateral reflections untreated. I just hope I don't need more bass absorption to get close enough seat-to-seat for EQ to seal the deal.