“as soon as the venue fills with people, it’s acoustics change”
You are spot on there. In my cinema presentation days as a projectionist we had a cue sheet for each film. Notes on the individual film with details of play out music etc. Included was the fader level for a typical average sized audience arrived at during several sound checks. If the auditorium was nearly empty you would reduce the fader number assigned. If it was nearer a full house, the fader number would be increased, not because the audience makes more ambient noise, but because people and clothing absorbs sound. During each showing the projectionist on duty would also by habit and for good showmanship make a light, focus and sound check in the auditorium during the first reel.
With regard to auditorium design, my regional theatre in Plymouth has a ceiling that can be lowered to the base height of the third tier to change the acoustic reflections for certain programmes avoiding the Royal Albert Hall effect. It facilitates a more controlled and intimate sound field.

