1. Supplies and returns are placed high or low based on the *primary* use of the HVAC system. Dual mode placement doesn't work and can totally screw with static pressures in your system.
2. Residential HVAC contractors (in J-School that is called a glittering generality ... I call it fact of life) don't have a clue about the special use requirements of a reasonably air tight, well insulated dedicated home theater...*including* consistently ignoring the requirements (ASHRAE) for fresh air exchange. It is your job to educate them (and while doing so, remember it is your room and your money, not theirs.)
3. If you tell a residential HVAC contractor the system must be designed to an NC20 requirement, in all likelihood they won't know what you're talking about (granted, I've been surprised a few times).
4. If you engage an outside mechanical engineer to design the HVAC system, the vast majority of residential HVAC contractors will use phrases like "you really don't need to do that" (the hidden meaning of that is "I don't want to be responsible if it doesn't meet spec", or "I'm going to charge you five times the normal rate to install this mess", or "I don't want to do this job because I have to maintain something I don't understand". Also, "you don't need a dehumidifier, the Air Conditioner will do that" (air conditioners are very inefficient at dehumidification and with high humidity, very inefficient at cooling to boot).
5. Residential HVAC contractors know more about HVAC systems, cooling/heating needs than anyone else on planet earth so whatever whomever told you x, y, or z is BS.
6. The location of supplies returns is very important and very often overlooked. For example, I've seen an HVAC contractor determine the "designer" was an idiot and since returns in the back of the room/supplies in the front would be "inconvenient", they installed the returns behind an acoustically transparent screen (now called an air filter). That cost the HVAC contractor $15K to "do over". You don't want cool air blowing down the necks of the people in the seats for two hours.
7. "This is White Horse (Adak, Barrows, Unalakeet, Northern Vermont) and you don't need A/C." Yes you do. End of Discussion. Full Stop. The method of cooling may not be by means of a compressor, but cooling you need. (If you use an air exchanger, use a right proper one that will provide filtering and humidity control.)
8. If you are going to stuff your room with eight seats, your cooling needs will be way beyond what you (or your residential HVAC contractor) would initially expect. ("...but, it will only be the wife and I except for about four times a year when we invite friends over." Ok. Fine. Why are you then spending $6K to $12K for excess seats that will be used four times in a year rather than getting a, say, better projector?
So, how do you deal with these guys? Very simply. You tell them the specifications they must meet (which includes where supplies/returns should be installed and what they can, and cannot do with respect to your overall sound isolation.) "Look, bud, I don't care how you do it, I don't care what you use, you won't affect my ceiling height, you won't degrade my isolation envelope, and when you're done, I'm going to stick a $2000 microphone 1 meter from that supply and if it doesn't meet NC20, you're paying to rip it out and start over again. Questions?"