Plaintiff's attorneys can name anyone even remotely related to the incident, no matter how far-fetched it may seem. You can't put liability in neat little containers like that, it's not what happens in the real world. Anyone even loosely connected to the incident with insurance and/or material wealth can and most likely will be named.
PA falls over? Well, the venue for sure gets named. Did anyone else touch the PA? Who? Was anyone else near the PA? Who was using it? Did they fail to notice it was not set firmly and may fall? Isn't the user of the equipment charged with some responsibility to ensure it is safe for him and everyone around him?
We're way beyond just the venue now. Several parties will be faced with defending themselves against liability.