I think Vanden Plas is the obvious answer for headliner. Many have stated them, and I think they are perfect for PPUSA. Not only are they "proggy" enough to satisfy the prog fans, but they have strong choruses with big hooks that are "sing along" if you will, as well as plenty of crunchy guitar work via Stephan that should appeal to many of the power fans. I would also love to see Urban do some more lethal mic stand tricks fronting Bloodound as well.
Those two would be a nice prog/power combination, I think.......
Bryant