John,

The line out shouldn't change with the volume adjustment--it should remain constant. The headphone out, at least with most keyboards, is directly effected by the main volume control. There is an input adjustment on the Zoom-H2 for both the mic and the Line-In.

When vocals come into play, the only option using the KMA, unless the vocals were fed through the keyboard's auxiliary input, would be to use the Bose L1 Compact's Line-Out, which is pretty hot, and adjust the H2's input down to about 50 percent. This would prevent clipping and distortion during the recording, while at the same time still provide a recording identical to what the audience is hearing, and without audience background noises. The H2's mics are somewhat omni-directional and relatively long ranged, therefore, they'll pick up just about every noise in the room.

Good Luck,

Gary