OK. I know all of you have been on the edge of your seats waiting for my findings. Here they are -- and I think I've stumbled onto a somewhat unexpected answer, at least in this case. I'll try not to get too long-winded in my report...
Firstly, what it was not:
CEILING FAN -- While the operating ceiling fan did produce some weird sound reflections, particularly with the treble strings, it was not the source of the 'muddiness.'
DAMAGE -- There was a period of time where I began to wonder if something had happened to the guitar. Did I jar something loose while transporting it? Was there a manufacturing defect (e.g., weakly glued bracing) that has only now beginning to surface? I frankly had to wonder... Thankfully, that did not seem to end up being the case.
HUMIDITY -- The humidity in my San Antonio playing area, according to the hygrometer I purchased around the beginning of this thread, has averaged about 50% RH. On the high end of the target zone, but perfectly acceptable.
Some of the contributing factors:
PICK -- My preferred pick (Dunlop .73mm ULTEX Sharp), which I have yet to replace, seems to produces a brighter sound. I've continued to use the 'generic' substitute pick I had on hand in an effort to reduce the number of variables leading to the answer. Ultimately, the pick was almost a non-contributor, but that may be because the substitute pick has similar properties to my preferred pick.
STRINGS -- The strings were definitely old, and I'm sure contributed to the dull-sounding guitar. But they were not the source of this particular problem.
STRUMMING TECHNIQUE -- Lumped with this I'll throw in guitar positioning. I found that, probably as a result of playing my GS 12-strings more frequently, I was letting my GA-bodied guitars sit a bit lower in my lap. As a result, I think I was not evenly strumming all of the strings, in particular the treble strings. I'd also thought that, perhaps, I was strumming a bit too hard for my GA Makore; my 12-strings seem to like a bit more thrashing. But ultimately that really didn't seem to enter into the solution. As a side note, it seems that each guitar I have has a strumming 'sweet spot' that makes it sound best. For a given body type, it's more-or-less in the same spot, so muscle memory helps one to keep playing there. But when switching between body styles (e.g., GS to GA), it requires a bit of re-calibrating.
The culprit:
ROOM ACOUSTICS -- When I got back into San Antonio, I took a few days to play my newly-strung GA Makore in exactly the same spot in the great room of my house. Not really any improvement that I could perceive; it still sounded a bit flat, 'boomy,' or 'muddy.' I tried swiveling around on my stool to see if pointing in another direction helped. Not really. Over the past several years, that area of the great room had accumulated a bit of clutter; mostly empty boxes and packing materials from things I'd bought, and that I had become lazy in either properly storing or disposing (forgive me; I am my mother's child, so I have a tendency to save these things in case I ever need to return or transport the item -- and my career has turned me into something of a nomad...). So after a few days, I gathered up all the clutter and moved it out of the room entirely. This expanded the playing area to the point where I decided to reposition my playing spot to the corner, back to the walls, and play toward the open room. When I pulled out my GA Makore, something remarkable happened -- I started hearing the guitar that I'd bought again: clear and well-balanced. My best guess is that my original playing spot in the room was just coincidentally located at a point of destructive acoustic interference, and my bright-sounding GA Makore was particularly susceptible to it. That spot just seemed to kill a lot of the treble response. I tried moving back to the original spot to confirm it and it does seem to be the case, even without the room clutter (although I think it was worse with all that absorptive cardboard and styrofoam).
So there you have it kiddies! If you don't like the way your guitar sounds, try cleaning house and moving to a new spot...!