So, I've played around with some potential layouts and ideas, and I think I'm converging on one of two options that are actually pretty similar, except for their orientation.Option 1:
This option keeps the screen wall on the North wall, as originally planned, and draws heavily from Tedd's suggestion (thanks!). The L/R speakers are 9 feet apart and partially fit into the alcove. I wasn't sure what a typical depth for speakers was, so I assumed 2 feet (and 1 ft wide) and then tacked on another 6" to the screen. Presumably if speakers are not that deep, I could move the screen a little closer to the North wall. The seating position (one row) is 11 feet from the screen to where your head would be. I've assumed as 120" wide 2.35 AT screen, which might be a little big, but I figured I could size that out a little more accurately once I had some of the other variables figured out. If it gets smaller, however, the L/R speakers would be outside (or partially outside the screen) - is that a problem? I spaced the speakers to have a 45 degree separation with respect to the seating position.
As you can see, the side surrounds are a bit of a sticking point. I put in two different options in terms of alignment for them, but I assume they will be symmetrical. The left surround is shown in a column on a short wall extending off of the planned hallway wall. This is a preferred location from the standpoint of the rest of the basement layout, but looks a little tight in the theater. How far should the surround speakers be from the seats? Also, I wasn't sure what dimensions were appropriate for those speakers either, so as shown, they are 1 foot deep and 2 feet wide. Perhaps an in-wall speaker rather than a column would open things up a little more? The right surround speaker is also shown in a column, but is further away by virtue of having some additional horizontal space in that part of the basement. That wall could be brought out to make it symmetrical, of course (or the column extended or something). Seems like there is less of an issue on the right side.
The West "wall" could potentially be moved out some, but there is a pole that is dictating some of the other wall positions in the basement (shown in the South-West corner of the theater). While most of that side would be open, I was planning to run a soffit to help define the West edge of the space (and possibly hang a curtain as Jeff suggested if that could help improve sound quality).
I didn't draw any rear channel speakers. From some of the numbers I ran, it seems like there is not enough distance from the seating position to the back wall (about 2.5 feet, I believe) to have much separation for those channels (60 degrees)? Will in-ceiling speakers improve that? Should I just accept a wider-than-optimal separation as a consequence of the room? How will that impact the sound from those channels? It may be possible to move the South wall back a foot or two, but that is not preferable. This narrower distance probably eliminates any form of second row seating, also, unfortunately.Option 2:
This option rotates the room 90 degrees, and I am really falling in love with this approach (thanks again Tedd!). Again, the L/R speakers are shown 9' apart, 30" from the East wall to the screen position, and 11' from the screen to the seating position. As shown, the screen is a retractable 2.35 120" AT screen. This approach leaves a lot of flexibility on some form of additional seating and I hope with full side walls, greatly improves the audio possibilities. I'm not sure what to do with the alcove and whether leaving it open will cause sound problems, etc, but that seems like a minor issue at this point.
There are two main sticking points with this option. First, a minor one - there is a window in the wall in the area where the center speaker would go. Since I do not believe I need to have access to this escape window, I think either some kind of blackout shade/tinting or window plug should address any light issues. The window is just under 3' from the floor. I don't think it will cause any problems with mounting the center speaker because the speaker should be completely in front of it, but thought I would mention it.
Second, a major (and potential deal-killer) sticking point is that there is an electrical panel
(which will need to have a sub-panel placed on one side or the other when we finish the basement) pretty much right behind the left channel speaker. I've been wracking my brain all weekend trying to figure out how I can go with this orientation option and still allow relatively easy access to the panel. If it helps, here's an actual photo of the space, towards the North East corner (you can just see the pole in the bottom right of the picture):
Any thoughts? Can the left speaker be mounted in such a way that it is easily moveable? Additionally, if I did a screen wall (which I would prefer over the retractable screen), is there some way to move the screen out of the way and/or get behind the wall easily? I've seen a few threads discussing hinged screens, but even that isn't complete, as there is still the lower portion of the wall in the way. Maybe curtains on the edges and not having the screen wall go all the way to the North wall? I'm not sure if that will leave enough room to get to the panel. The panel has to be accessible - I'm sure code requires it and also, if a breaker trips, I don't want someone stumbling around in the theater in the dark trying to get to it and walking through the screen
As always, thanks again for everyone that has chimed in so far, your help has been invaluable, and more input is greatly appreciated!