Again, I'm very grateful for the help, and I hate to do this, but I'm going to revive this thread, as I have a big weekend in store.
Taking delivery of my 65VT30 tomorrow, and will run the slides as suggested for at least 100 hours. In the meantime, I'd like to get some wire fishing and "prework" done while I can for my Polk surrounds (Monitor 30s).
With that being said, I'll post some new images below, and would like some feedback on the proposed locations.
I didn't want to clog the picture up more than it already is, so here is a little color legend. The blue is where today's crappy Onkyos are. I'm now considering the green areas, as that will help spread my spacing out (roughly from the 8' I have today to almost 12.5') and will have them at a somewhat even height and depth. My big concern with this area is the doorway down to the utility room/basement. Does that protrusion hurt the effect of the speakers? Will this "depth" be a detriment, as opposed to having the speakers up flush with that doorway to the basement?
As much as I'd like to mount these way outside where I had originally hoped, I have come to realization that it just isn't feasible, and there is no way to get the wires out there cleanly (or cleanly based on my capabilities). Therefore, I think I'm stuck getting something through the encasement for the ductwork and the I-beam.
And finally, the red circle is a random idea I had for the sub (Klipsch RW-12d). I know the sub crawl and all that entails, I'm just curious if this cubby hole would even be sufficient. It's intended for firewood, but is kind of a lost space. After some measuring, I realized it would fit in there with sufficient (I think?) breathable space. I'd have about 6" of extra space on top, 6" of extra space on each side, and about 5" of space behind it if I put the grill somewhat flush with the opening. And there is power in there, and I have a plan to get the cable over there. My main concern is it is plywood finished on the inside, so how "boomy" would it be? Is that too tight of an enclosure?
Here are the updated pics. One with my new colors, and one unedited, since my updates make an already dark picture even harder to comprehend: