Designing a stereo sound control

I have been reviving SoundField using Processing. I am still pleased with the simple and intuitive audio interface, but what bugs me is the design of the stereo controls.

Especially with multiple overlapping sound areas, the stereo circles create "circle noise" - it becomes difficult so see which circle belongs to which sound.

Time for a little research for alternative designs.

The following diagrams show the sound area move control in red, and the stereo sound control in blue.
At the left side the left and right channels are in balance; at the right the audio is panned halfway the right.

The current stereo control:

Possible alternatives...

The stereo sound as a masked background:

As a curved line segment, with a smaller circle as control:

The curved line segment as draggable control:

A small draggable line inside (below) the move control:

I like the clean look of the last design best. But because of the small size the affordance needs some work.