Kamelot did two North American runs on their tour already, and they're going into the studio, so they're out. Children of Bodom had shows planned up until summer when they imploded, so perhaps they were negotiating something like this for the fall. Sonata Arctica just did one NA run last fall, and they have scheduled Europe, Finland, South America and festivals all until this fall, so perhaps it was them and they decided to take a break instead. And Amorphis is on the exact same schedule, and they've been doing a lot of co-headlining tours recently, so I'd put my money on them.