Originally Posted by gareth_alien There are no support poles marked on that plan
- if that's true then great, but it would be unusual.
I just wanted to +1 this comment and make sure it registers. I have 21' spans in my room. It took additional engineering and cost to get there. You have 28' spans. There isn't a snowballs chance... that they intend to make those spans without poles unless you've addressed it already.
If you haven't discussed support poles or structural support beams with your architect, you need to ASAP.
Also consider how HVAC, pipes, and utility lines will be run through the space. What will that do to your ceiling height? Will it cause any irregularities or protrusions into the room? I would highly recommend incorporating the a routing plan in your design and put it in the contract that the builder has to follow it. Have the architect or designer reroute as much as possible around the theater.
Consider your truss design. Do you want try to run things through the truss webbing, or use use metal or wooden I-beams. I did open web trusses and it didn't turn out as well as I had hoped mainly due to lack of route planning. But it still netted me ~12' ceilings. If I had it to do over, I would probably try conventional metal beams and dig down further to make up for the loss in ceiling height. But digging down can be expensive, especially if you hit rock which is typically lumped in with the "act of god" exclusions to the contract price. The more you dig, the more you roll the dice and if you hit a lot of rock it can be cost prohibitive to continue to dig.
You also need to consider the proximity of noise generating sources to your theater. Place your mechanical rooms far away from your theater room so you don't have to deal with the noise. Also think about where your external A/C compressors will sit relative to the theater.
If you want to sound proof your floor/ceilings you need to mass up the floor over the theater. The architect needs to take into account the additional thickness of the floor and drop the support walls (and poles) an inch or two so the top of your finished floor is all one level. Trying to retrofit ceiling soundproofing later within the theater is expensive, labor intensive, and not nearly as effective.
Watch the move "Mr Nobody" with your wife. It's on youtube for free. Don't rush. Get everything in writing, and put every last detail in the contract. If everyone remains friends to the end, no harm done. If things go south, it's all you've got.