I think that all the bands you mentioned have one or more elements of Prog in their sound, but they musically differ..of course they do, all bands within a genre will..
But as long as they incorporate some of the elements below they could qualify as prog
Longer songs, ranging to epic length
Fantasy elements and spiritual themes
Symphonic sounds
Complex time signatures
Classically influenced elements
Concept albums
Theatrical performance
However I do take exception to the fact that Metal bands are described as Prog when they feature longer songs or have complex timings. Its a definition of Metal that it is a distilled form of Blues, simpler and harder. It contradicts prog, in the same way Punk does. 'Fraid I am a real purist when it comes to Prog.
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