I’m not sure how "progressive rock" is defined.
So, I am going to be 'that guy', and bring up the "what is prog" discussion.
For me, prog, is not a style of music (it's barely a genre), in the same way country, blues, punk, etc are.
Prog has more to do with the structure of the music, than any sort of surface veneer and style the music may have.
One could play just a few example songs of the above genres (country, punk, blues), and get a pretty good idea about how the majority of the rest of the genre sounds. Sure, there are differences between interpretations, but at their core, they have their attributes that define them. Blues for example, has to be in minor pentatonic scales, or it is no longer blues.
But what could someone nail down about how prog sounds, by playing something from: Yes, Gentle Giant, National Health, Magma, Univers Zero, Can, Area, Pain of Salvation, Anglagard, Samla Mammas Manna, Bruford, Gong, Il Balletto di Bronzo, Henry Cow, etc?
These bands, literally sound almost nothing like each other.
The thing that defines all the above bands as prog, for me, are all or most of the following attributes, in no particular order: a very high level of musicianship, complexity (chord progressions, time signatures, arrangements, syncopation, etc), deep and broad levels of emotional content, (sometimes) long format pieces, nonstandard song format, avoidance of verse>chorus>bridge song format, avoidance of catchy hooks, influences from disparate types of music (classical, jazz, folk, Eastern), sometimes use of dissonance.
There is no specific sound a band has to have to be prog, but there are certain other attributes (those above), a band has to have to be prog.
For example, the modern technical-metal bands, Cynic, Tesseract, The Contortionist, sound almost nothing like prog bands from the 70's, but they are certainly prog. Ridiculously high level of musicianship, very complex, very emotional, nonstandard song formats, jazz-fusion influences, are all there.
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