I am a young musician, I have been listening to what most people considered the most important band to the future of progressive rock the Beatles. While I like Pink Floyd, The Doors and Traffic. The early music to me that has influenced has been the Beatles progressive rock stuff. I don't think they should be classed as proto-prog becuase they are not easily piegoned holed. They went from Merseybeat to folk rock to what I call psychedelic progressive. These songs could be the roots of many genres in progressive rock.
Norwegian Wood- Raga Folk
Tomorrow Never Knows- Experimental rock
Strawberry Fields Forever, A Day in the Life and I am The Walrus- Avant symphonic rock
Love You To, Within You Without You and Inner Light- Indian Progressive Rock
Happiness Is A Warm Gun
Here Comes The Sun- Synth Folk Rock
I Want You She So Heavy- Synth Hard Rock
Side Two Abbey Road- Symphonic Rock