I'm glad you say the low ceiling is not a disaster. A lot of things have sounded quite amazing, so perhaps I've just figured out where the low ceiling factor becomes a major issue, untreated.
I will try to learn REW, and HolmImpulse also. It's helpful to hear what you're using the software for, as I don't really know how to go about with analysis. This is probably why folks keep telling me to contact GIK — because there's a lot to this, and my fumbling around isn't going to get me that far. Still, knowing what I can use a *few* things the tools for to help my room (and speaker choice) a little bit is something I can seek out for the moment. I can at least seek to learn something about the major problems getting in the way of the test speakers' having a fair hearing. I have already taken some steps to soften the rooms reflection points, though I have not used measurement to know exactly how it helps. In this way, I am effectuating your comment about not trying to match "speaker to room or room to speaker is," though I am not yet on the way toward a scientific target.
Regarding this basement's floor, the room has a concrete floor with thin wall to wall carpeting, but also several thicker area rugs, other furniture (couch, leather chair, bed, wooden chest covered with a blanket), so there's quite a range of absorbtion and diffusion going on.
Regarding ceiling reflections, I may look into replacing the blanket and getting an OC73 absorber but I'd hope I can just mount it temporarily without getting into a DIY project at the start. Given that I'm not sure how far apart the speakers should be, ultimately, I'm not sure whether or not 4ft X 8ft will cover the correct area, though maybe there's a lot of wiggle room in something 8 feet across?
Bass traps — noted. I'm not finding any problems with the bass that make me want to set up traps *before* making a speaker choice. Still, if as you say there could be issues with frequencies *above* the bass which are affected by peaks or nulls, then some home-made experimentation may be in order. I appreciate you mentioning some targets to shoot for, such as 400 ms and T60. I bought a miniDSP mic that I can put at my listening position or at other places in the room (I obviously need to read about how to go about this), and see if I can figure out the software.
FYI, the rear wall is actually a built in bookshelves above cabinets that run the entire length of the room, with books of various heights all along. The two side walls are far away though I will confess that the speakers are not the same distance from either side wall. A bit hard to explain. My electrical setup has limits which necessitate this arrangement.
Anyway, super helpful post. Thank you.
@jtcf Good luck with your search, too!