Mike, Im sending out a larger piece of damascus for you to use on the handle spine. Should be there by the end of the week. Barring postal delays. lol
Seeings how you asked. I would stay with the black wood and use the damascus for the spine and spacers. The Koa and the horn are to similar in color and may not give enough contrast or definition to the amazing handle you designed. Im sure that whatever you go with it will be great.