Wow, four pages back..... You've really got to start posting more pictures
I've been thinking about HVAC a lot recently, and it seems to be a popular topic here on the boards. I had forgotten just how big your room is! Those 12' walls really add to the volume. Looks like you're going to have over 7,000 c.f. once you account for risers, soffits, stages, seats, etc. etc. So you're going to need roughly 700 cfm flowing into and out of your space to get the oft-quoted 6 exchanges per hour.
Now, I don't remember how many people this theater is planned to seat, but assuming your shooting for 8, that's going to require 4,000 BTU/hour of heat removal. A rule of thumb for sizing HVAC is 400 cfm/Ton, so you can figure you need about 135 cfm of cooled air from your HVAC to keep the room cool. Another rule of thumb here is 15-17cfm per person to keep the space cool (both rules of the thumb give about the same results
That's a pretty large discrepancy between the needed cooled air and the amount of air you need to exchange. Here's a chart I found for sizing HVAC lines, and it looks like you could get by with a single 8" supply from the HVAC. Maybe even go up to a 9" to be safe.
With all those open trusses, you could easily add some dead vents to exchange air with the other rooms in the basement. Panasonic makes a 440 cfm inline fan that's supposed to be super quiet. Here's a link if you're interested
. If you go with the 9" supply, and add the 440 cfm dead vent, on paper you're at 665 cfm. Obviously the actual installation might be a little lower, but that's pretty dang close.
As far as velocity goes, again, an oft-quoted requirement is 250 fpm. If you go with the 9" supply from the HVAC, you can up the size to a 10" a couple feet before you enter your theater to slow down the air. Note, you can't run a 10" all the way and get the same result. You need a restriction to limit the flow to what you want, and then an expansion in the duct size to slow down the flow. It's still going to be too fast with the 10" duct at around 400 fpm, but you can build a register with a bar diffuser on it to slow it down the rest of the way. The other option would be to use a 12" or 14" going into the theater or hidden in the soffits which should get you right at that 250 fpm mark.
The dead vents would be similar. If you split the 8" from the inline fan into two lines before entering the theater or in your soffits, you'll have about the same flow in each as the HVAC supply.
Sorry for posting the novel here, but I've been reading a lot about this lately, and thought it might help. Feel free to tell me to stop cluttering your thread if you like