Well, I know this won't be a popular move: I've decided to scrap the double bass array!
Reality has set in, and I've realized that the seats need to be moved as far back as possible in order to allow for reasonable viewing angles as well as appropriate acoustical treatment. Therefore, the riser has to butt up against the rear wall and partially cover what would have been the rear speaker mounts.
The good news, I hope, is this: it won't matter. My sub response is pretty good up to about 60 hz (see post #22), and likely will will improve once I add lots of bass trapping. I think this response represents that of the entire basement, as I currently have no door and lots of open space above the rear speaker box. So, my plan is to "let the bass out" of the theater. That is, drill lots of large holes through the MDF of the (former) rear speaker box and cover them with very thin material, such as 1/8" luan or 1/4" drywall. I'll have to experiment to find out what works best.
Speaking of bass trapping, I'm probably going to stuff the entire rear speaker box with R30, creating a bass trap that's about 32" deep, 7 feet tall, and about 9 feet wide. I'll add lots of side/ceiling fiberglass and helmholtz resonators (either tuned pipes or membrane traps) where needed. My plan is to go for a very dead room and use electronics to make up for the lost ambience. I'll get into LEDE (live end dead end) theory, ISD (initial signal delay), ambience, etc. later when I finally get to acoustical analysis.
Here's my riser progress as of now. I'll mount 2 more 15" Elemental Designs 160 v2 subs in the front of the riser and leave the back hollow, to be filled with bass trapping- fiberglass and or helmholtz resonators as needed. Check out the home brew clamp jig I concocted to get my frame level- that worked out really well.
The riser will be covered with cherry wood and will support two "zero gravity" Relax the Back chairs. If any of you have back problems, these chairs are a must have. I've never fallen asleep in any other chairs but these.