Win7 64 GTX 770
I use an addon to be able to use [Add] blend mode strips for Additive animation layers.
Adjusting the keyframe values usually work, however when there are no NLA strips in the animation, the active action will only interpret values as though the action_blend_mode is set to REPLACE.
If you push the track to NLA, or just add a track (and mute it), the animation's keys will be properly read for "adding"
This Blend file contains a rig with keyframes on the left arm, and an action strip with no keyframes in the NLA.
If you delete the empty strip, the keyframed bones will switch from adding 0 (keyframed value) to the Scales (which base is 1), and go to setting the Scale to 0.
When the poses break, if you push the action to tracks, then reset the transforms (without keyframe insertion), the properties will go back to the intended values.