With the exception of a few bands, hard rock in the sixties and the seventies can be summed up as the blues with the substitution of simple meter for compound meter and the loss of the 12-bar format. The lion's share of tunes from that period contain simple i-iv-v minor chord progressions with basic chord substitutions/additions and pentatonic minor scale-based solos. It wasn't until Randy Rhoads and Yngwie Malmsteen entered the stage that rock music took a hard right with respect to fusing classical with rock to form the style that pretty much dominated heavy rock in the eighties; namely, neoclassical fusion. While the chord progressions and rhythm (meter) remained relatively simple, fast diatonic minor-based guitar solos with chromatic and modal playing mixed in for good measure dominated the landscape. Before the eighties, only Uli Jon Roth and Ritchie Blackmore where skilled at using chromatic and modal diatonic passages in hard rock.
There's no hiding the classical influence in tunes like Mr. Crowley. It was the first radio-friendly really heavy tune to showcase the classical hard turn that music was about to take. https://www.youtube.com/watch?v=3D_9FIYZQ5c
Before the release of Blizzard of Oz, the heaviest classical influenced tune that I heard was "Sails of Charon" by the Scorpions. Uli Jon has amazing control on this tune. It was released before Van Halen's debut album shook the hard rock world like a 7.0 magnitude earthquake.http://www.youtube.com/watch?v=aoI7deS76Ck
With that said, since there appears to be a group of people on this forum that are around my age. Has anyone ever heard of a band called Crack The Sky? Although Crack The Sky formed in Weirton, WV (a town west of Pittsburgh), they were Led Zeppelin huge in Maryland. Their music is hard to define other than it was very progressive for the era in which they recorded it. Crack The Sky's first four albums (Crack The Sky, Animal Notes, Safety in Numbers, and Live Sky) are their best. The album White Music is good, but it was recorded by a subset of the original band. Crack The Sky performed a cool version of the "William Tell Overture" on Live Sky. They also wrote about subjects that were taboo during the period (e.g., "She's a Dancer," http://www.youtube.com/watch?v=o9AQa_1frBI
). One of their funniest tunes is "We Got Mine," which was written about the bad of a deal that they cut with Lifesong Records (http://www.youtube.com/watch?v=5UL7M9jUXZI
). The line "We haven't seen silver since the airplane crashed last year" is a reference to Jim Croce's death. Jim was the marquee name on Lifesong Records. A lineup that includes quite a few of the original members performs from time to time. They can still rock for a bunch of old guys.