Just as Dennis says.
More specifically, the source resistor is chosen to set the quiescent current through the JFet pair to about 60% to 80% of the Idss. First the JFets are matched as buffer pairs, then for corresponding stereo locations. In a buffer pair, the JFet with slightly higher Idss is used in the top position. The source resistors may be chosen to keep quiescent currents approximately the same, and may be adjusted over a range of values. (The pinch-off voltage of the JFets may also affect this, but this is above my level of understanding.)
There have been recent discussions in several threads, including the ACP+ thread, as well as the B1 Korg thread.