Hey Glenn,
I would just like to post some of my thoughts as a Progpower USA virgin. (first year baby!!!) I am one of those who've been aware of this festival for a number of years, but just haven't made it down to Atlanta for whatever reason (usually because of tuition, bills, etc.) I have to say that the draw for me this is year is 2 fold:
1. I think this is the best US festival, period! Now I haven't been yet, but as has been stated before, you consistently have a fantastic lineup and the fans are never disappointed. I'm proud to become a contributor to the vision that you have! It is refreshing to see someone put together a concert with nothing in mind but sheer musical integrity. You bring the best to the fans, and that is freaking rad! My Bolt Thrower t-shirt says it best..."In a world of compromise, some don't", and that is how I feel about this festival. It needs to be supported, and I for one am happy to do so!
2. The second thing that draws me this year is the *quote* "undercard" bands. I'm from Utah, and I learned a long time ago that to see the best bands, i'd have to travel out of state. With that said, i've seen or had the opportunity to see most of the "headliners" that you've had at Progpower over the years, but there are quite a number of bands on your opening to 4th or 5th slot that I haven't seen, and that is my draw to the festival this year. The metal fans in this country can see Symphony X, Sonata Arctica, or Helloween every couple of years either right in, or near their hometown. They have a harder time however seeing Vanden Plas, Sabaton, or Orphaned Land.
I guess what it boils down to for me is rarity. You produce what others don't.
If it is ticket sales that you are after, then I don't think that there is anything wrong with scoring an already "on tour" band, or a more mainstream band as a headliner as long as you have some good diversity and rarity in the lineup (as you consistently do). We have a lot of good US based bands that would work pretty well.
This show seems to pull a lot of "out of towners", and I think that that is awesome. If that is to continue, then the rarity factor has to stay. Luckily, as has been stated on this thread, that rarity needn't be necessarily from a headliner to get people's attention.