They don't really do a lot of harmonies though. As far as vocals go, they really don't do anything in the harmony department, even JLB has said that before. And instrumentally they're much more into guitar/keyboard unisons than harmonies. Not that they never do them, but it's really rare, and it's pretty much a third apart every time. Dream Theater is much more about straightforward melodies with complicated rhythms and subtleties going on underneath.
Opeth do a lot more, and imo more interesting, harmonies. I haven't heard a lot of Pain of Salvation but they seem to do quite a bit from what I've heard, maybe the actual PoS fans on this board could correct me there. Even Symphony X did a lot more in the way of harmonies, both instrumentally and vocally, back when they had a more progressive/symphonic edge.