I think some early Fleetwood Mac Qualifies.
Oh Well, Parts I and II for instance.
Renaissance's "Trip to the Fair".
Superb piece of music. Annie Haslam is another stunning vocalist.
Focus is a Dutch progressive rock band formed in Amsterdam in 1969 by keyboardist, vocalist, and flautist Thijs van Leer, drummer Hans Cleuver, bassist Martijn Dresden, and guitarist Jan Akkerman.
One more from the late 90's-early aughts. From a Dream Theater album about what someone learns from a past-life regression. The album is brutally powerful, and this cut is achingly beautiful. I've literally listened to the album four times in the last couple of days.
and the studio, rather than the stage,From Wiki: the style was an emergence of psychedelic bands who abandoned standard pop traditions in favour of instrumentation and compositional techniques more frequently associated with jazz, folk, or classical music. Additional elements contributed to its "progressive" label: lyrics were more poetic, technology was harnessed for new sounds, music approached the condition of "art", and the studio, rather than the stage, became the focus of musical activity, which often involved creating music for listening rather than dancing.
So Yeah, Boston and Styx dabbled in the Prog Rock. They were also considered Arena Rock.
Having read and listened my way through loads of wonderful music here, much new to me, I might offer another dozen answers. Is the Lee Michaels and Frosty album below prog rock? Chicago, Jethro Tull?Yeah, and ask ten people what Progressive Rock is and you'll get, well at least 2-3 different answers.
Genesis - Carpet Crawlers: