I've been listening to Rush's Presto album, which is not on my high rotation list, along with some of their other 80's albums...
But after listening to Show Don't Tell enough times, it seems pretty obvious there were some cool jam sessions with all three guys that created the wicked bass lines Geddy plays against Neil's drumming. They captured the ingredients and vibe of the jam into that song.
Other Rush songs that come to mind are Freewill and YYZ. Early Black Sabbath also had that feel of jam sessions churning out cohesive songs.
What other bands, bassists and songs spring to mind where it seems obvious there were some major jam sessions behind the songs?