Transatlantic my friend:
Permit me introduce yourself: Iīm son of 70īs too, I grew in the 80īs hard rock ( and why not ) heavy gardens, so I discovered the progressive rock music in the early 90īs. For me it was a big discovery.
Accourding the definitions, progressive music can be all long duration musics, that sound as concept musics; various climate situations inside the same song; complexity in arrangement and in musicians performance; triping lyrics.
I think that under this definition, Dream Theater is prog as Rush. In my point of view, although I donīt like Dream Theater music.
So to close my reply, try this link: Is very elucidative, and it will help you to understand the Progressive Rock sub-genres, and space, art, fusion, symphonic, rock.
http://www.avila.med.br/closeto/arvore.html
Truly yours,
Vanila