There is so much brilliant music that I believe falls within what you are describing.
Obviously, a very high percentage of prog from all eras fits. And of course, the use of Mellotron by a high percentage of prog bands also lends that orchestral feel, with the Mellotron’s analog sampling of massed orchestral: strings, horns, flutes choirs, oboes, etc is a big contributor to the orchestral fee.
Yes, King Crimson, Genesis, Gentle Giant, Renaissance, are a few from the classic rock era.
But other countries also produced some world class symphonic/orchestral prog, also.
Italy being the best and most prolific. But France and Spain have their share, too. Check out:
Premiata Forneria Marconi (PFM) / their first 4 or so releases are incredible. Monstrous musicianship, beautiful melodies.
Banco del Mutuo Socorrso (Banco) / Keyboard playing right up there with Wakeman or Emerson, great compositions, and an incredible vocalist with close to 5 octave range.
Arti E Mestieri / Their debut album, Tilt, is like symphonic prog meets jazz-rock fusion. With terrifying chops by all band members.
Änglagård / From, Sweden. Loads of orchestral sounding Mellotron, Beautiful, dark melodies, quite complex.
The Flower Kings / Sweden. A pretty traditional prog band (Yes, Genesis influences), very prolific, with plenty of great material. Stardust We Are, Flower Power, and Unfold the Future are probably their strongest releases. Their latest releases are a bit too mainstream for my tastes.
I could talk about quite a bit of the prog subgenre of avant-prog, which has a great deal of symphonic sounding material, but bands in this subgenre compose mostly atonal music, so it can be a bit "thorny" sounding. And not always an easy listening experience, but artistically rewarding nonetheless.
These bands also tend to have a lot of non-traditional (for rock) instruments that give them an orchestral feel: cello, violin, oboe, flute, English horn, and others.
