Trying to put sound into words is very difficult.
Here's my attempt:
You have heard violins you like and violins that you don't like. If you can mentally define the sound of a fiddle you don't like, you have koa's worst qualities.
It's not like other tone woods; koa doesn't fit all sizes and/or types of guitars equally well. It tends to favor some sizes over others and some construction. types over others.
The wood works best on ukuleles because uses are high-pitched and use nylon strings. Their pitch and the strings both work to gentle koa's worst qualities. A small all-koa guitar with a slightly thick top can sound all right too
To me, an all-Koa guitar is a violin I may not like. The sound is usually a bit over-bright, verging toward harsh, with a lot of harmonic overload that is not pleasant to my ears. For as hard as it is, koa doesn't sustain as much as it should, so the tone is quite brittle and short.
But that's not always true. When the guitar's top is not koa, the wood makes for a very bright guitar with less bass than maple, but the sound may not be unpleasant at all.
A soft top like redwood can smooth out koa's tone and can add some good bass and low-kids to it. So can cedar and any of the softer spruces. Even mahogany could do well teamed with koa.
When there's a strong contrast between the top and a koa body, with the workmanship intended to bring out the sound, and doesn't follow the same crafting as a typical guitar, koa can be quite good-sounding. I think it would be better as a rhythm guitar than as a lead instrument though.
The wood is visually gorgeous for sure, when it's at its best. It's ironic I find that the prettier the wood is, the more I dislike the sound. But when the koa is quite plain and has little figure, the wood always sounds better.
