The thing about psychedelic rock that makes it so interesting and proto-prog
is the sonic (sound texture) element of it. You could say that this even bordered
on suggesting the avant-garde or "sound-art" type music, as it probably was the genre of the three
elements of proto-prog, said very correctly above, that added that aspect to
prog rock. I very much like the idea of proto-prog being all three: psychedelic,
baroque, and jazz-rock.
It's interesting that wider arrangements had already been
going on in "middle of the road" type music from the early 60s. There are
many people today who collect these "anomalous" records from thrift shops,
so they are no longer completely obscure. It would be interesting to hear
what was happening before "Pet Sounds" hit the market. When you get
trained big band or orchestra musicians backing up pop singers, more than
the typical blues-scale stuff can happen.
I know some German schlagel (spelling?) had some interesting
stuff going on. I'm not sure the date it was happening, but it was
pop music with orchestration. I guess back then it was a crossover between
pop and opera/classical song structures, and symphonic arrangements.
I don't know when Ina Martell's "Ich war allein" was released, but you
can see that maybe the Beatles heard stuff like this and did Eleanor Rigby.
If you also look at all the changes in some songs by singers in the Sinatra generation,
they were pop but sometimes complex. There may have been a progressive wing
in the more popular music of the 20s and 30s. Check out, "It Was a Very Good Year,"
by Sinatra.