Demoiselle said
I was just pondering the point of not hearing fellow musicians enough.

This is an issue that can make me very clinging. Which happened when I practiced with a guitar player a couple times in March and April. Usually she chose a place where I couldn't hear her well enough, but I followed her and placed myself where her sound opening was pointing at my face. In a session it's always possible to move around while playing and place my ear close to a guitar or keyboard instrument.

I perform with two little speakers, which I fix on my self-made violin stand. Actually, I had planned to place the speakers directly on the timber of the stage for my long concert, since the stage resonates extremely well, which produces a very warm and bassy sound. But I will probably not do that, for on the violin stand the speakers point upwards to my face. Otherwise I will turn on the speakers too loud.  

In bluegrass jams, players typically sit or stand in a circle (the smaller the group, the tighter the circle), which serves two purposes: players can hear each other, which helps with vocal harmony, and see each other, which helps everyone know the chord progression. If you know what guitar chord shapes look like, you can tell what chord is being played.

That said, when the group is large it's sometimes it is hard to hear the singer or the person taking a break (solo), despite everyone's efforts to play softly. The cumulative volume of that many instruments is still loud. That's why I've come to prefer smaller groups.