Then the A/V guy decided that something wasn't right. He lowered the lighting so the screen showing the PowerPoint was brighter. And the speaker was dimmer. Satisfied, he left the room.
I'm not going make the assumption that all A/V guys (or gals) only care about the technology in the room, but it was certainly the case with this man. He only saw the screen; it was all that mattered to him. The speaker in the dark? Who needs a speaker when you have a PowerPoint on the screen?
This is, unfortunately, a common occurrence. Don't let it happen to you! Take charge of the lighting in the room the way you take charge of the room layout and non-technological aspects of your presentation. You may not have many options for lighting, but still check it in advance, set it the way you want and if someone tries to change it, intervene.
The audience must clearly see the speaker; that's where the presentation is happening. Ideally, they can see your slides, too, and the lighting should work for both the speaker and the screen.
But think of it this way: If your computer broke down and you didn't have your slides, would you still be able to give your presentation? The answer should be "Yes, of course!" The slides are peripheral to your presentation, they are not THE presentation. Don't let an A/V guy (or anyone else) make your slides the star.