I saw "Epitaph" May 15 in Orland Park, Illinois (Chicago suburb). We had about the same amount of people attending as you report. There was no advertising, so you had to be a hardcore Judas Priest fan to know about the movie.
The sound was initially too low, but a member of our audience asked the staff to turn the volume up. The experience was much better with everything cranked to eleven, even though the speakers would distort every now and then. Movie theatres can't handle Ian Hill!
The overall experience was great, though. How often do you get to see a Priest concert film in theatres? There were actually a few people in the audience chanting, "Oh, yeah!" with Halford. Of course, the concert was awesome. Not only was the music excellent, but the interaction within the band was fun to watch.
I last saw Judas Priest live in Hammond, Indiana during the Epitaph tour, and this movie was a really nice complement to that show (which I actually think was better). Now fans have the the Blu-ray, the new album and some possible live dates to look forward to. It's very gratifying to see a band that's been around for over forty years continue to produce new material and still help us all get our Heavy Metal fix.