Recently I've been listening a bit to one of my favorite '70s prog bands: Gentle Giant. When I first discovered this band as a college student I went out and bought every one of their albums in 3 weeks (and it only took that long because I had to special order most of them). The band members all sang and played at least 30 different instruments between them, often trading mid-song! Music was diverse, ranging from Renaissance to 20th C. classical to hard rock ... and beautifully complex. You can find many great videos of theirs on YouTube. Here is one of my favorites: On Reflection, beginning with a rather classical arrangement, then a capella singing, then a good hard rock treatment of the same themes.