Being a huge fan of 70`s American soft-hard rock, prog-pomp rock, makes me a huge fan of groups like Kansas, Styx, Journey, Toto and Boston etc but they were very much groups of their era and by the mid to late 80`s that sound had largely died away, for years I looked around for similiar sounding groups with an updated sound but never found anybody. Point being, that when Dream Theater and Fates Warning arrived they kind of filled the vacum that had been left by these groups, they all had obvious influences from the previous groups and all had the musical ability that these groups were associated with, along with great vocalists and musicians and they also the commercial appeal (less so in FW case) The only difference is, that both DT and FW had a metal feel, it was almost like they had been airbrushed with metal coloured paint for lack of a better phrase, and the hey! all of sudden you were listening to Toto or Kansas with a metal feel!
Albums like Masques, Leftoverture and Point of Know Return were classic and intricate albums and you can here all this in DT`s early work.
As for Opeth, well its like here is a proper heavy metal group, heavy and aggressive with a heavy dosage of death metal, lengthy compositions and dynamic shifts, I know this could be argued as not being enough to make them progressive metal and could easily open a debate as to what is progressive metal, my only point is, that I see Opeth as HM and DT not so based on DT obvious musical influences.