Is prog rock genre defined by instrumental sounds?

This is where it gets hard. I'm not sure there is a proper venue for what you want, specially if we were to place this within a context of musical history. The compositional side of it, would tend to create something new that would defy this and that and convention specially. 

I like to think of Stravinsky, when he first came out and how different he was. It was not about a "sound", it was about the whole work, and I think that you are likely stuck on a keyboard sound that you think defines "prog rock", and that's insane ... you can go buy most of those keyboards these days at the local music store, or on the Internet ... and creativity, has absolutely NOTHING to do with an instrument, or a sound ... it has to do with its continuity and total design.

Hope this helps, and I'm sorry it is simplistic.