@ghosthouse, Yeah, that's a good one. Also on Pet Sounds is the sublime "God Only Knows", one of my three all-time favorite songs. It has not only strings and brass, but also woodwinds. And unlike The Beatles who required George Martin to write the parts for the orchestra instruments, Brian wrote his himself (he reads and knows music notation). "God Only Knows" has three bass parts---one for electric (Carol Kaye playing an incredible part containing numerous examples of inversions, my absolute favorite thing in music. It was hearing that part that woke-up Paul McCartney to the possibilities of the instrument), one for string (upright) bass, and a third for the Fender-6, a really cool 6-string bass that Duane Eddy played guitar solos on. Just above the basses is a cello.
Brian's aborted Smile album is what I think Progressive Rock can be, and mostly isn't. Informed by Classical (actually, using your knowledge of music theory in the writing of songs is a better way to put it), but not attempting (with poor results, imo) to combine it with Rock, or play it with a Rock band. That's just silly, and sounds that way. Pictures At An Exhibition by ELP is just dreadful. It was written for solo piano (I have a recording of it so performed), and orchestrated by Maurice Ravel. THAT is how it should be performed, not by a 3-piece Rock band. That can not possibly do justice to the music.