How can I Change the Default Volume of an Instrument in Kontakt?

You can change this in the Instrument Options of the instrument.

1. Load the instrument, then click the Wrench icon on the left to open all controls.
2. Click the first button on the top left labeled Instrument Options, then click Controller.
3. You can set the parameter labeled MIDI Controller #7 (Volume) Range, this will enable you to set the default volume range. The higher of the two numbers here will be the default volume when the instrument is loaded.
4. To permanently save your setting as the new default, remember to save your instrument before closing.