I used to have a bass made from koa, but it was twenty years ago and I'd really be stretching my memory to remember its tone.
Here's what I found at Taylor Guitars website:
"A tropical hardwood, koa's tone blends the midrange of mahogany with the top end of maple. Due to its density, a new koa guitar tends to start out sounding a little bright and tight, somewhat like maple. But the more a koa guitar is played, the more the sound opens up, expanding the midrange and rewarding the player with a richer, sweeter, more resonant tone. A common mistake is when a bright player buys a koa guitar in part for its visual beauty, finds it to be too bright, and doesn't play it enough to allow the wood to warm up."
If this is true, and it does become more middy and less bright over time, a Miracle Man bridge pickup might do the trick for your application as it generally works well with maple and middy guitars.
This might only be the case for acoustics though ...
